FIX: MakeEscapedJsonString now null-checks inputs

[stuartbrown-unity]

Description

Most device decriptor fields are null-checked in ComparePropertyToDeviceDescriptor, however the addition of MakeEscapedJsonString meant that 'capabilities' was not being checked anymore, and so could throw a null-ref exception if a device didn't fill this field.

Changes made

This change adds a null/empty string check to MakeEscapedJsonString to mirror the old behaviour.
Additionally this'll also now mean any devices with an empty non-null string can skip a minor StringBuilder allocation.

Testing

Local testing with gamepads to see that the resulting comparisons in ComparePropertyToDeviceDescriptor still work correctly. This was tested between null, empty, and valid json string inputs.

A new unit test has been added too to exercise the null-input path.
Unit tests were also ran.

Risk

Minor difference in construction of a default JsonString (with a non-null text field).

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • JIRA ticket linked, example (case %%). If it is a private issue, just add the case ID without a link.
  • Jira port for the next release set as "Resolved".
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

After merge:

• Create forward/backward port if needed. If you are blocked from creating a forward port now please add a task to ISX-1444.

[unity-cla-assistant]

All committers have signed the CLA.

[ekcoh]

LGTM, if there is an associated public issue tracker please link it to the CHANGELOG row (See other examples). Additional feedback from you @AlexTyrer ?

[ekcoh]

Seems like branch has some conflicts that need to be resolved.

[ekcoh]

Conflicts on branch