Bug-2017254: Handle null signatures for PerfCompareResults

[moijes12]

Background

• Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=2017254
• In PerfCompare, opening the subtests view for certain results would result in a Server 500 Error being displayed. However, the error was being gracefully handled by PerfCompare and no crashes or traceback was displayed to the end user.

Analysis

• Calls from PerfCompare to Treeherder's perfcompare/results api endpoint were returning Server Error 500
• On debugging further, it was observed that this was reproducible if either the base_parent_signature or new_parent_signature parameter was set to null
• The calls to _get_signatures were throwing exceptions as new_parent_signature was set to null in the caller.
• The PerfCompareResultsQueryParamsSerializer allows null to be passed for signatures. The signatures are defined as CharFields. @Archaeopteryx @camd : Why is the field created as Charfield while the signatures in other serializers for the Performance data are defined as Integerfield ? What was the reasoning behind it ?
• Filtering on null results in an error as it expects as integer. The null is also not mapped to None.

Proposed Fix

Added validators to map null to None if null is passed for base or parent signatures in api call to PerfCompareResults.

Fixes Bug-2017254

@Archaeopteryx @kala-moz @gmierz @beatrice-acasandrei