Automatically fix implicit None for mypy using official tooling

By running:
pipx run no_implicit_optional flair
pipx run no_implicit_optional tests

Following:
https://github.com/hauntsaninja/no_implicit_optional

flair/data.py

@@ -313,7 +313,7 @@ def to(self, device: str, pin_memory: bool = False):
                 else:
                     self._embeddings[name] = vector.to(device, non_blocking=True)
 
-    def clear_embeddings(self, embedding_names: List[str] = None):
+    def clear_embeddings(self, embedding_names: Optional[List[str]] = None):
         if embedding_names is None:
             self._embeddings = {}
         else:
@@ -352,12 +352,12 @@ def remove_labels(self, typename: str):
         if typename in self.annotation_layers.keys():
             del self.annotation_layers[typename]
 
-    def get_label(self, label_type: str = None, zero_tag_value="O"):
+    def get_label(self, label_type: Optional[str] = None, zero_tag_value="O"):
         if len(self.get_labels(label_type)) == 0:
             return Label(self, zero_tag_value)
         return self.get_labels(label_type)[0]
 
-    def get_labels(self, typename: str = None):
+    def get_labels(self, typename: Optional[str] = None):
         if typename is None:
             return self.labels
 
@@ -472,7 +472,7 @@ class Token(_PartOfSentence):
     def __init__(
         self,
         text: str,
-        head_id: int = None,
+        head_id: Optional[int] = None,
         whitespace_after: int = 1,
         start_position: int = 0,
         sentence=None,
@@ -682,7 +682,7 @@ def __init__(
         self,
         text: Union[str, List[str], List[Token]],
         use_tokenizer: Union[bool, Tokenizer] = True,
-        language_code: str = None,
+        language_code: Optional[str] = None,
         start_position: int = 0,
     ):
         """Class to hold all metadata related to a text.
@@ -831,7 +831,7 @@ def to(self, device: str, pin_memory: bool = False):
         for token in self:
             token.to(device, pin_memory)
 
-    def clear_embeddings(self, embedding_names: List[str] = None):
+    def clear_embeddings(self, embedding_names: Optional[List[str]] = None):
         super().clear_embeddings(embedding_names)
 
         # clear token embeddings
@@ -946,7 +946,7 @@ def to_original_text(self) -> str:
             [t.text + t.whitespace_after * " " for t in self.tokens]
         ).strip()
 
-    def to_dict(self, tag_type: str = None):
+    def to_dict(self, tag_type: Optional[str] = None):
         labels = []
 
         if tag_type:
@@ -1100,7 +1100,7 @@ def set_context_for_sentences(cls, sentences: List["Sentence"]) -> None:
                 previous_sentence._next_sentence = sentence
             previous_sentence = sentence
 
-    def get_labels(self, label_type: str = None):
+    def get_labels(self, label_type: Optional[str] = None):
         # if no label if specified, return all labels
         if label_type is None:
             return sorted(self.labels)
@@ -1138,7 +1138,7 @@ def to(self, device: str, pin_memory: bool = False):
         self.first.to(device, pin_memory)
         self.second.to(device, pin_memory)
 
-    def clear_embeddings(self, embedding_names: List[str] = None):
+    def clear_embeddings(self, embedding_names: Optional[List[str]] = None):
         self.first.clear_embeddings(embedding_names)
         self.second.clear_embeddings(embedding_names)
 
@@ -1362,7 +1362,7 @@ def _downsample_to_proportion(dataset: Dataset, proportion: float):
         splits = randomly_split_into_two_datasets(dataset, sampled_size)
         return splits[0]
 
-    def obtain_statistics(self, label_type: str = None, pretty_print: bool = True) -> Union[dict, str]:
+    def obtain_statistics(self, label_type: Optional[str] = None, pretty_print: bool = True) -> Union[dict, str]:
         """Print statistics about the class distribution and sentence sizes.
 
         only labels of sentences are taken into account

flair/datasets/base.py

@@ -1,7 +1,7 @@
 import logging
 from abc import abstractmethod
 from pathlib import Path
-from typing import Generic, List, Union
+from typing import Generic, List, Optional, Union
 
 import torch.utils.data.dataloader
 from deprecated import deprecated
@@ -110,7 +110,7 @@ def __init__(
         database: str,
         collection: str,
         text_field: str,
-        categories_field: List[str] = None,
+        categories_field: Optional[List[str]] = None,
         max_tokens_per_doc: int = -1,
         max_chars_per_doc: int = -1,
         tokenizer: Tokenizer = SegtokTokenizer(),