Crash during parsing in indexes

[BagritsevichStepan]

We should not crash during parsing different options in FT.* commands. For example: (empty separator in FT.CREATE)

127.0.0.1:6379> ft.create index on json schema $.sometag AS sometag TAG SEPARATOR
Error: Server closed the connection

So we should add parsing tests for all FT.* commands, especially for the FT.CREATE, FT.SEARCH and FT.AGGREGATE commands.

• Fix the crash with empty separator #4208
• Add tests for different options in the FT.CREATE command
• Add tests for different options in the FT.SEARCH command
• Add tests for different options in the FT.AGGREGATE command
• Test all other FT.* commands