Preserve empty-string env() defaults and support nested function-calls

[edalzell]

Problem

FunctionWriter::convertToArgs() silently drops any argument that's an empty string, treating it the same as an omitted one:

$f->env('SOME_PATH', '');
// renders as: env('SOME_PATH')   — the '' default is gone

That's indistinguishable from actually wanting env('SOME_PATH'), so there was no way to write a genuinely-empty default via the fluent API.

Separately, every argument was coerced through gettype()-based scalar conversion, which breaks the moment an argument is itself a built expression rather than a plain scalar:

$f->env('CONTENT_PATH', $f->basePath('content'));
// throws / mis-renders — there's no way to nest one function-call builder result inside another

That meant you couldn't build something like env('CONTENT_PATH', base_path('content')) through the fluent API at all — you had to drop into raw PhpParser node construction by hand.

Solution

• convertToArgs() now only treats an explicit null as "omit this argument" — empty strings pass through as real values.
• FunctionWriter::arg() now checks $arg instanceof Expr first and passes it straight through, instead of trying to scalar-convert it. This is what makes nesting work: $f->basePath('content') already returns a FuncCall (an Expr), so it now flows straight into the outer env() call's args.
• The single-arg path helpers (resourcePath, storagePath, basePath, etc.) now default to null instead of requiring an argument. That preserves the "call with no arguments" result (f()->resourcePath() → resource_path()) — previously the only way to get that was to pass '', which is exactly the behavior being fixed.

Testing

Three new cases in LaravelFunctionCallTest: an empty-string default is preserved, a path helper called with no arguments renders as a bare call, and a basePath() call nested as an env() default renders correctly. All pre-existing tests still pass (2 unrelated, pre-existing closure-formatting failures aside, confirmed present before this change too).