Fix variable handling and null values

[NyanKiyoshi]

This pull request fixes two issues:

1 – InputObject variables were not handled

When providing an object for an InputObject, as a variable instead of hard-coding it into the query, we would get:

AttributeError: 'AttributeFilterInput' object has no attribute 'value'

Query

query($filters: AttributeFilterInput) {  # <-- the type is not handled
  ...
        availableAttributes(first: 10, filter: $filters) {
          edges {
            node {
              name
            }
          }
        }
  ...
}

Variables

{
  "filters": {
    "search": "color"
  }
}

2 – Null values were not handled

When providing a null value, we would get:

AttributeError: 'NoneType' object has no attribute 'value'

Query

Same as above.

Variables

{}

[codecov-io]

Codecov Report

Merging #27 into master will decrease coverage by 0.3%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage   97.07%   96.77%   -0.31%     
==========================================
  Files           6        6              
  Lines         308      310       +2     
==========================================
+ Hits          299      300       +1     
- Misses          9       10       +1

Impacted Files	Coverage Δ
graphene_django_optimizer/query.py	96.04% <100%> (-0.37%)	:arrow_down:

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