fix(expense): Expenses with no summary fields

[athewsey]

Issue #, if available: #370

Description of changes:

• Stop textractor.parsers.parser_analyze_expense_response from ignoring ExpenseDocuments where no summary fields were detected.
• Add a test case to verify that expense documents with no summary fields are not ignored.
• Add a warning in the event that page number can't be determined for an ExpenseDocument and it needs to be skipped: Since silent skipping was confusing for users & debugging.

Confirmed with customer from #370 / @arsher-b in call that they're okay for the shared document to be included as a test case.

I didn't bother creating an extra test case for the warning branch (page number could not be inferred) yet because:

1. It seemed tricky
   • Do we have any existing tests for whether logging.warning gets called?
   • I can't think of any image that would generate it, so think we'd need to just create a synthetic JSON
2. I don't think it was covered by previously existing tests anyway

...But guess we could revisit this if needed

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[Belval]

LGTM