MOD-6540: Support EMPTY indexing for TEXT fields

[raz-mon]

Describe the changes in the pull request

Currently we discard empty strings for TEXT fields in our index, i.e., we don't index them and don't allow querying for them as well.

This PR adds the support of indexing and querying for such empty strings ('') for TEXT fields.

This is done by adding the ISEMPTY keyword in the index creation for the desired TEXT field, as such:

FT.CREATE idx SCHEMA my_field TEXT ISEMPTY

Then, querying for such empty values is done via the isempty(@my_field) syntax in the query, as such:

FT.SEARCH idx "isempty(@myfield) ... "

This syntax is compatible with all other query constructs, just as any other first-class citizen.

Some cleaning and commenting was done as well,

Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

[codecov]

Codecov Report

Attention: Patch coverage is 89.55224% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 85.90%. Comparing base (c32a768) to head (781f44d).

Files	Patch %	Lines
src/spec.c	93.50%	5 Missing ⚠️
src/inverted_index.c	0.00%	3 Missing ⚠️
src/doc_table.c	50.00%	1 Missing ⚠️
src/document.c	80.00%	1 Missing ⚠️
src/json.c	66.66%	1 Missing ⚠️
src/numeric_index.c	50.00%	1 Missing ⚠️
src/query.c	91.66%	1 Missing ⚠️
src/query_optimizer.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                           @@
##           razmon-feature_empty_text_indexing    #4622      +/-   ##
======================================================================
+ Coverage                               85.79%   85.90%   +0.11%     
======================================================================
  Files                                     188      188              
  Lines                                   33121    33089      -32     
======================================================================
+ Hits                                    28417    28426       +9     
+ Misses                                   4704     4663      -41     

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[nafraf]

Good. Some comments

[oshadmi]

🤩 Very nice! A few more comments.

[oshadmi]

⛏️ Minor comments