Fix part of #5070: Display empty answer message in Math expressions input interaction.

[Vishwajith-Shettigar]

Explanation

Fix part of #5070, In Math expressions Interaction UI, leave submit button enabled when answer is empty. Show an error on submitting an empty answer.

MathExpressionInteractionsViewTestActivity added to accessibility_label_exemptions and test_file_exemptions files.

Numeric expression

numerix_expression.webm

Algebraic expression

algebraic_expression.webm

Math equation

math_equation.webm

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
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[Vishwajith-Shettigar]

@adhiamboperes, PTAL.

[oppiabot]

Unassigning @Vishwajith-Shettigar since a re-review was requested. @Vishwajith-Shettigar, please make sure you have addressed all review comments. Thanks!

[BenHenning]

Thanks @Vishwajith-Shettigar! Took a pass and had just a few comments. PTAL.

Also, could you update the branch to the latest develop (as there are conflicts to resolve)?

[Vishwajith-Shettigar]

Thank you @BenHenning, PTAL.

[BenHenning]

Thanks @Vishwajith-Shettigar! Just one follow-up--PTAL.

[Vishwajith-Shettigar]

@BenHenning, PTAL.

[oppiabot]

Unassigning @Vishwajith-Shettigar since a re-review was requested. @Vishwajith-Shettigar, please make sure you have addressed all review comments. Thanks!

[BenHenning]

Thanks @Vishwajith-Shettigar! This LGTM.