Fixes #3145: ExplorationActivityTest failing tests fixed

[rt4914]

Explanation

Fixes #3145

The two tests were failing because the createVoiceoverMappingsFromJson was returning map for empty content-ids too.

This PR updates the implementation to check if voiceover/contentid json is empty then do not add that as Voiceover to the map.

Screenshot

In current develop the audio icon is available in toolbar in all explorations. The below screenshot shows that it is no more visible for for states which do not contain audio.

Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The PR explanation includes the words "Fixes #bugnum: ..." (or "Fixes part of #bugnum" if the PR only partially fixes an issue).
• The PR follows the style guide.
• The PR does not contain any unnecessary auto-generated code from Android Studio.
• The PR is made from a branch that's not called "develop".
• The PR is made from a branch that is up-to-date with "develop".
• The PR's branch is based on "develop" and not on any other branch.
• The PR is assigned to an appropriate reviewer in both the Assignees and the Reviewers sections.

[BenHenning]

Thanks @rt4914 & nice catch. I would ask for a test to be added, but given this is temporary code I think time is better spent not bothering if ExplorationActivityTest confirms the fix.

Just have 1 comment to address--PTAL.

[BenHenning]

Thanks @rt4914. LGTM!