server: fix ComStmtSendLongData when data length is 0

[jackysp]

What problem does this PR solve?

setCharacterStream(1, new StringReader(""), 0); this API in JDBC will use ComStmtSendLongData to send a long string. When the data length is 0, TiDB cannot distinguish whether there is a parameter with no data or there is no parameter at all.

What is changed and how it works?

This pr appends an extra 0 at the end of the bound parameter. In ComStmtExecute, we use it to distinguish if there is a parameter or not.
(How does MySQL deal with this?
MySQL set the length of the parameter as len(data - 6), which 6 is the length of the header of the request.)

Check List

Tests

• Integration test (JDBC test cases)

Code changes

• Has exported function/method change

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

PTAL @coocood

[zimulala]

LGTM

[winkyao]

LGTM

[jackysp]

/run-all-tests

[coocood]

@jackysp

This is a way to solve the problem, but may not the best.
The problem is that a nil slice appending a zero-length slice is still nil slice.

We can make it work like this in TiDBStatement

	if len(data) == 0 {
		ts.boundParams[paramID] = []byte{}
	} else {
		ts.boundParams[paramID] = append(ts.boundParams[paramID], data...)
	}

[jackysp]

/run-all-tests

[jackysp]

PTAL @coocood

[coocood]

LGTM