fix: build_filter_conditions utils method

[aMahanna]

The current implementation of arango.utils.build_filter_conditions() does not support filtering by None values (see the test failures triggered by 75d7d7a)

This method is being used across many of the "simple" methods as of #275

A simple fix is to rely on json.dumps to serialize the arbitrary values into a JSON formatted string

[codecov-commenter]

Codecov Report

Merging #277 (3b048b5) into main (744a6e9) will decrease coverage by 0.24%.
Report is 11 commits behind head on main.
The diff coverage is 92.38%.

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@@ Coverage Diff @@ ## main #277 +/- ## ========================================== - Coverage 99.01% 98.77% -0.24%  ========================================== Files 26 26 Lines 3958 4013 +55 ========================================== + Hits 3919 3964 +45  - Misses 39 49 +10 

Files Changed	Coverage Δ
arango/client.py	98.41% <ø> (ø)
arango/http.py	100.00% <ø> (ø)
arango/collection.py	97.59% <91.39%> (-0.92%)	⬇️
arango/exceptions.py	100.00% <100.00%> (ø)
arango/request.py	94.44% <100.00%> (ø)
arango/utils.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[apetenchea]

LGTM