forms: x-blankAs "empty" -- a string cleared in an edit form submits "" instead of being omitted

[Yaraslaut]

What / why

DynamicForm leaves a blank control out of the payload. Consider an edit form prefilled from a stored record, where the model reads an absent std::optional<std::string> as "leave unchanged" and "" as "clear". In that form a prefilled string cannot be cleared: when the user deletes the text, the member is omitted and the stored value stays.

Contract

Schema key: property-level "x-blankAs": "empty", read with the same dual-read as the other property keys. The descriptor exposes it as blankAsEmpty. It is only read for fields of kind string, including ["string","null"] and the nullable anyOf spelling.

What a blank control submits, by field state since the last prefill / resetFields:

Field state	Blank control submits
Prefilled with a string, "" included	"field": ""
Non-blank at any point via any write path (control, setFieldValue, slot setValue), then cleared	"field": ""
Never prefilled and never non-blank. A stored null does not count as prefilled.	omitted, as before

Other rules:

• Required fields: unchanged. A required field left blank is still unfilled, so the form is not ready.
• Non-string kinds (number, enum, Choice, datetime, …) ignore the key.
• C++: FieldMeta gains BlankAs blankAs{BlankAs::Omit} (enum class BlankAs { Omit, Empty }) plus withBlankAs(). It emits "x-blankAs": "empty", but only on a std::string / std::optional<std::string> member.
• Wire: unchanged. The key only affects presentation.

The engagement rule has one consequence to be aware of. In a create form, typing into such a field and then clearing it also submits "": the user did touch the field. Only a field that was never touched is omitted.

Tests

• New src/qt/forms/tests/tst_DynamicFormBlankAs.qml, 12 cases, all asserting the submitted body. It covers:
  • a prefilled field that is cleared submits ""
  • an untouched field is omitted
  • a field typed into and then cleared submits ""
  • a stored "" round-trips; a stored null does not engage the field
  • a field without the key is still omitted
  • a non-string field ignores the key
  • a required field is still gated
  • both reset and re-prefill disengage the field
  • a slot's setValue("") behaves the same
• New tests/test_forms_blank_as.cpp checks that the key is emitted on string members, and not by default or on a double, and that the wire is unchanged.

Mutation checks (actually run):

Break	Red
A cleared field never submits ""	4 of 12
Engagement ignored, so an untouched field also submits ""	6 of 12

Local results (MSVC / Qt 6.11.1, offscreen):

• forms_qml_logic: 409 passed, 0 failed
• morph_tests "[forms]": all passed

Independent of #822 / #823.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
include/morph/forms/forms.hpp	42.85%	4 Missing ⚠️

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