[wasm][debugger] Correctly handle empty, and whitespace-only strings (#41424)

[wasm][debugger] Correctly handle empty, and whitespace-only strings (#41424)

There are two cases being fixed here:

1. str=='', or str=' '
	- We check `str_value == 0`, and for the above cases JS returns
	true, due to type coercion.
	- So, we show the result as a null string.

2. str==null
	- debugger.c adds the value for this with `mono_wasm_add_typed_value ("string", NULL, 0)`
	- the second argument is converted to a string with
	  `Module.UTF8ToString(..)`, but when it's `0`/NULL, we get an
	  empty string. And that becomes a null string, because of (1).

Fixing this by using `===` operator to avoid type coercion.

Fixes https://github.com/dotnet/runtime/issues/41276