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