MOD-7206: [2.6] Fix sorting multiple fields

[GuyAv46]

Describe the changes in the pull request

Fixing sorting by multiple keys, when the two rows are missing the primary(s) key(s) but hold a value for secondary(s).

Consider the case where doc1 = {docID = 1, n = 93}, doc2 = {docID = 2, n = 46}.
We want to sort by a missing field as the primary sort, and by n as a secondary.
... SORTBY 2 @missing @n ...
Since both are missing a value for missing, we should treat them as equals and continue to compare them according to the secondary n key. We get that doc2 is the first and doc1 is the second (doc1.n > doc2.n)

Before the fix, we would see that they both are missing missing and we will immediately fall back to comparing them by docID (our last resort). In this case we get that doc1 is the first and doc2 is the second (doc1.docID < doc2.docID)

Which issues this PR fixes

1. [BUG] FT.AGGREGATE Inconsistent Multi-Sorting in Redis 6.0.20 with RedisSearch v2.6.12 #4711
2. sorter simplification #3501 - Original fix for master, affecting 2.8 and forward
3. MOD-7206

Only relevant for the 2.6 branch.

Mark if applicable

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.65%. Comparing base (8ffd0a3) to head (8222de5).

Additional details and impacted files

@@            Coverage Diff             @@
##              2.6    #4717      +/-   ##
==========================================
- Coverage   83.65%   83.65%   -0.01%     
==========================================
  Files         176      176              
  Lines       31147    31146       -1     
==========================================
- Hits        26057    26056       -1     
  Misses       5090     5090              

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