Return an empty array from Utf8.encode for an empty string #406

The array-returning cpp.encoding.Utf8.encode returns null for an empty string. It should return a non-null array of length zero, so callers can use the result without treating empty text as missing data.

Reproduced against current main at 5f7cfef66ed4d9543c6c1a2763bdc2148433435c, using Haxe nightly 5.0.0-preview.1+fe63b35 with runtime API 500, MSVC 19.44.35213 on Windows x64 and GCC 13.3 on Linux x86-64 under WSL. Builds use HXCPP_M64, default GC settings and smart strings.

var bytes = cpp.encoding.Utf8.encode("");
trace(bytes == null); // true

The empty-string branch of Utf8::encode(const String&) returns literal 0 from an Array<uint8_t> function. That constructs a null array. The overload that writes into a caller-provided view correctly returns a byte count of zero; that return value does not work for the array-returning overload.

Return a real empty byte array and keep null-input validation separate. This is a runtime-only fix.

A conventional regression is prepared in test/regression/Utf8EmptyArray/{Main.hx,build.hxml,stdout.txt}. Expected output has all three checks true. Actual output:

empty UTF8 encoding returns an empty array: false
ASCII UTF8 encoding remains intact: true
Unicode UTF8 encoding remains intact: true

class Main {
	static function main() {
		var empty = true;
		var ascii = true;
		var unicode = true;
		#if (haxe_ver >= 5 && !haxe_5_preview_1)
		var bytes = cpp.encoding.Utf8.encode("");
		empty = bytes != null && bytes.length == 0;
		var plain = cpp.encoding.Utf8.encode("abc");
		ascii = plain != null && haxe.io.Bytes.ofData(plain).toString() == "abc";
		var wide = cpp.encoding.Utf8.encode("\u0100");
		unicode = wide != null && haxe.io.Bytes.ofData(wide).toString() == "\u0100";
		#end
		Sys.println("empty UTF8 encoding returns an empty array: " + empty);
		Sys.println("ASCII UTF8 encoding remains intact: " + ascii);
		Sys.println("Unicode UTF8 encoding remains intact: " + unicode);
	}
}

Build with the Haxe nightly using haxe -main Main -cpp bin -D HXCPP_M64, then run bin/Main (bin/Main.exe on Windows). This exercises API 500; older compiler configurations skip the new encoding API.