fix: leave out a braced-list member initialiser that has no value [patch]

[matt-edmondson]

Summary

new MemberInitialiser("x") has a null value. That is legal, and the graph editor holds exactly this between adding a member initialiser and filling it in. Every braced-list writer fell back to new VariableReference(string.Empty) for a null value, and that constructor throws on an empty name. So all seven generators threw ArgumentException: Name cannot be null or whitespace, and the preview broke mid-edit.

Following the triage note, the element is skipped:

• New LanguageGeneratorBase.WrittenElements(construction) returns the list's elements without any MemberInitialiser { Value: null }.
• The shared StandardLanguageGenerator.WriteElementList / WriteStackedList (C, C++, Go, Rust), CSharpGenerator.WriteBracedList, PythonGenerator.WriteLiteral / WriteArguments and JavaScriptGenerator.WriteLiteral now write those elements.
• Every caller now filters first, so the ?? new VariableReference(string.Empty) fallbacks could no longer be reached and are removed.
• The shape of the list (designated initialiser, object initialiser, keyword arguments, object literal) is still decided by every argument. A list whose only element is unfilled therefore stays the designated form: Point{} in Go, new Point { } in C#, {} in Python.

The same fix also covers the Go, Rust and C constructor bodies that route initialisers through these list writers.

Example, Point { unfilled = <none>, y = 2 }:

Target	Output
C	Point p = { .y = 2 };
C++	Point p = Point{ .y = 2 };
C#	Point p = new Point { y = 2 };
Go	var p Point = Point{y: 2}
Rust	let mut p: Point = Point { y: 2 };
Python	p = Point(y=2)
JavaScript	let p = new Point({ y: 2 });

Out of scope, possible follow-up: the Python and JavaScript constructor initialiser paths (WriteInitialiserAssignments) don't throw. On a null value they write self.x = / this.x = ; instead.

Tests

• New UnfilledMemberInitialiserTests, run against all 7 generators:
  • UnfilledMember_IsLeftOut: the unfilled member is left out and its filled sibling is still written.
  • OnlyElementUnfilled_IsAnEmptyList: a typeless list whose only element is unfilled.
• On main, all 14 cases fail with ArgumentException: Name cannot be null or whitespace. With this change they pass.
• Full Coder.Test suite: 1085 passed, 0 failed.

Fixes #112

🤖 Generated with Claude Code

https://claude.ai/code/session_017USYdQkEy3VLP48Dd4JadP

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
96.7% Coverage on New Code
0.0% Duplication on New Code

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