[BUG] A clear Exception/Error message must be thrown while trying to create a Sentence object from an empty string

[bharatr21]

Describe the bug
If one tries to create a Sentence object from an empty string, an UnboundLocalError of the index variable is thrown

To Reproduce

>>> from flair.data import Sentence
>>> s = ''
>>> Sentence(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\XYZ\AppData\Local\Programs\Python\Python36\lib\site-packages\flair\data.py", line 338, in __init__
    index += 1
UnboundLocalError: local variable 'index' referenced before assignment

Expected behavior
Something like ValueError: Cannot create a Sentence object from an empty string or some Error or Exception mentioning that the string is empty or a Sentence object "" - 0 tokens

Environment (please complete the following information):
I feel this is applicable to any OS/version, but anyway:

• Windows 10
• flair-0.4.1
• Python 3.6.8

[bharatr21]

I believe that the if text is not None condition could be changed to if text here to catch the empty string, but I need to know if this could break code elsewhere.
@alanakbik If this is a go, I'm willing to submit a PR 👍