Fix FTS5 tokenizer to prevent treating null character as token character

[yannaingtun]

Description
Fix FTS5 tokenizer to prevent treating null character as token character
This PR fixes a security issue in the FTS5 tokenizer where a null character ('\0') could be incorrectly treated as a token character. This same issue was fixed in the SQLite main repository.
The fix ensures that the null character is explicitly marked as not being a token character by adding a single line that sets aAscii[0] = 0.

References:

• Similar fix in SQLite: sqlite/sqlite@d1d43ef
• Commit message from original fix: "Prevent fts5 tokenizer unicode61 from considering '\0' to be a token characters, even if other characters of class "Cc" are."
• Related vulnerability: CVE-2023-36811

[levlam]

The character "\0" is never present in the strings for which TDLib uses FTS5, hence the upstream issue is irrelevant for TDLib.

[yannaingtun]

I understand that TDLib doesn't process strings with null characters in FTS5, making this issue irrelevant for your implementation. Thank you for reviewing and providing this clarification.