Fix VTT issues

[dsavinov-actionengine]

Problem and/or solution

VTT fixes for issues found during the 1st QA pass

How to test

Reviewer checklist

Code:

• Change is covered by unit-tests
• Code is well documented, well styled and is following best practices
• Performance issues have been taken under consideration
• Errors and other edge-cases are handled properly

PR:

• Problem and/or solution are well-explained
• Commits have been squashed so that each one has a clear purpose
• Commits have a proper commit message according to TEM

[dsavinov-actionengine]

Implemented issue 6 from QA spreadsheet: system should fail if source content consists of only WEBVTT signature

[dsavinov-actionengine]

Implemented issue 3 from QA spreadsheet: system should allow uploading (parsing) even if some subtitles are empty

[dsavinov-actionengine]

@kbairak
Important note below for issue 1 ("when the stringset sent to the compiler has no strings, the source strings should be added instead to that stringset.")

In short: At openformats level, we don't have info during compile() about the source strings. The fix might be appliet in transifex code.
Details:
When Transifex calls parse method, the parse(content) makes a template (where hashes replace strings) and a stringset from the content.
Then, when Transifex calls our compile(template, stringset) method, only the new translated stringset is passed to us, but we don't get the original stringset or at least the original content in the compile() method. It seems only Transifex has access to everything at once: source content, source stringset, template, translated stringset.

Please let us know your thoughts on this.

[dsavinov-actionengine]

@kbairak added fixes for AE-45 + checked for scenarios you mentioned:

1. during parsing, if the string that's about to be extracted is empty, don't include it in the resulting stringset at all
2. during compiling, if the stringset has an empty string, the resulting file should have this string's section but with the string content empty
3. during compiling, if a string is missing from the stringset, the resulting file should not have the string's section at all (the timestampts and anything else should be removed)

Testable were 1 and 2. 3 is only testable outside openformats testbed

[kbairak]

Hey @dsavinov-actionengine, thanks for the update. I will start checking.

This

3 is only testable outside openformats testbed

however, is not true. You can do/write a test like this:

handler = VttHandler()
template, stringset = handler.parse(...)  # A source text with 3 strings
compiled = handler.compile(template, stringset[:2])  # skip the 3rd string
assertEqual(compiled, ...)