Fix part of #5070: In FractionInteraction UI, leave submit button enabled when answer is empty

[masclot]

Fix part of #5070: In FractionInteraction UI, leave submit button enabled when answer is empty. Show an error on submitting an empty answer. The error message already exists and is the same as in web.

Demo video: leave_submit_button_enabled_on_empty_answer_v2.webm

Explanation

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[adhiamboperes]

@masclot, this approach looks okay to me.

It would also be nice to see an example of how you will handle the specific messages listed in oppia/design-team#71 (comment)

[adhiamboperes]

Suggested change

	if (answerText.isNotEmpty()) {
	pendingAnswerError =
	FractionParsingUiError.createFromParsingError(
	fractionParser.getRealTimeAnswerError(answerText.toString())
	).getErrorMessageFromStringRes(resourceHandler)
	} else {
	pendingAnswerError = null
	pendingAnswerError = if (answerText.isNotEmpty()) {
	FractionParsingUiError.createFromParsingError(
	fractionParser.getRealTimeAnswerError(answerText.toString())
	).getErrorMessageFromStringRes(resourceHandler)
	} else {
	null
	}
	}

[BenHenning]

Thanks @masclot. I think the solution in the video also looks good, and I took a quick scan at the implementation (but I'm mostly deferring to Adhiambo for that).

Could you also add tests for this change? I think either StateFragmentTest or StateFragmentLocalTest would be a good fit (based on whichever currently contains the tests for fraction input errors). It'd be useful to test the different new scenarios of where the submit button is enabled when empty.

[oppiabot]

Hi @masclot, I'm going to mark this PR as stale because it hasn't had any updates for 7 days. If no further activity occurs within 7 days, it will be automatically closed so that others can take up the issue.
If you are still working on this PR, please make a follow-up commit within 3 days (and submit it for review, if applicable). Please also let us know if you are stuck so we can help you!