Fix C# .NET 10 project creation failing with invalid CLI args

[Copilot]

Fixes project/function creation failures on .NET 10 caused by invalid CLI arguments being sent to dotnet new (--namespace and --FunctionsHttpPort "undefined").

Root causes

1. --FunctionsHttpPort "undefined" — StringPromptStep.getDefaultValue() called String(this._setting.defaultValue) even when defaultValue was undefined, producing the literal string "undefined". This pre-filled the prompt and was submitted verbatim.
2. --namespace "Company.Function" — DotnetFunctionCreateStep always added --namespace to dotnet new args, but newer .NET templates (e.g. the .NET 10 Azure Functions function template) don't declare namespace as a valid parameter, so the CLI rejects it.
3. Filtering — the arg filter in executeDotnetTemplateCommand (value !== undefined && value !== '') allowed null and stringified‑nullish values ("undefined", "null") through to the CLI.

Checklist

• Investigate issue and identify root cause
• Preserve whether the raw .NET template declares a namespace parameter (supportsNamespace flag on IFunctionTemplate, set in parseDotnetTemplates.ts)
• Gate --namespace in DotnetFunctionCreateStep on template.supportsNamespace && context.namespace; also skip null values when iterating userPromptedSettings
• Skip the namespace prompt in DotnetNamespaceStep when the template reports supportsNamespace === false
• Fix StringPromptStep.getDefaultValue to return undefined when the template has no defaultValue, instead of the literal string "undefined"
• Harden the arg filter in executeDotnetTemplateCommand.ts to exclude null and literal "undefined"/"null" strings
• Resolve merge conflict with origin/main (overlapping fix in StringPromptStep.ts from Fix SignalRTrigger C# template showing "null" as default Hub Name #4993)
• tsc --noEmit and eslint pass clean

[nturinski]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved in 36a7b8e. The only conflict was in StringPromptStep.ts — main's #4993 added the same defaultValue == null guard I had; I kept both the guard and my explanatory comment. tsc --noEmit and eslint pass clean.