Fix CanExecute(null) for value types without predicate

[apachezy]

Closes #1206

When RelayCommand<T>.CanExecute(object?) receives null for a non-nullable value type T, it unconditionally returns false — even when no canExecute predicate was provided. This violates the documented contract: "The default return value for the CanExecute method is true."

This single-line fix distinguishes between the two cases:

• No predicate → return true (the command is always executable, as documented)
• Has predicate → return false (cannot meaningfully evaluate null as T, safe fallback — unchanged from current behavior)

In WPF applications on .NET Framework, the framework may call CanExecute(null) during control initialization before the CommandParameter binding has produced a value. With the unconditional return false, commands without predicates are permanently disabled, as the initial false result is cached and never re-evaluated.

This fix is fully compatible with the reasoning in WindowsCommunityToolkit #3619: for commands with a predicate, mapping null to default(T) would conflate "no value" with a legitimate value (e.g., 0 as SelectedIndex). The conclusion that return false is safe for that scenario is preserved here. The case of no predicate was not explicitly considered in that discussion — a command without a predicate should simply always be executable, as the class documentation states.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• New component
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

Change in src/CommunityToolkit.Mvvm/Input/RelayCommand{T}.cs:

 if (parameter is null && default(T) is not null)
 {
-    return false;
+    return this.canExecute is null;
 }

Scenario	canExecute	Before	After
RelayCommand<int>(Do)	null	false ❌	true ✅
RelayCommand<int>(Do, i => i >= 0)	predicate	false	false (unchanged)

TryGetCommandArgument、ThrowArgumentExceptionForInvalidCommandArgument、Execute(object?) 均未改动。

[apachezy]

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[apachezy]

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