kernel/util: Accept inputs of 0 bytes in utf16_to_utf8.

There's no reason to reject them; they should just result in
a zero-byte output.

Should fix #20222.
(cherry picked from commit 4b6aad38968f9fe91712e742aff9166fec6e67a1)

Change-Id: Ifdbe17618d74eca0b873a6808ec0b14a7d38189a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/11402
Reviewed-by: waddlesplash <[email protected]>

src/system/kernel/util/convertutf.cpp

@@ -59,10 +59,8 @@ static ssize_t
 utf16_to_utf8(const uint16* source, size_t sourceCodeUnitCount, char* target,
 	size_t targetLength, bool isLittleEndian)
 {
-	if (source == NULL || sourceCodeUnitCount == 0
+	if (source == NULL || target == NULL || targetLength == 0)
-		|| target == NULL || targetLength == 0) {
 		return B_BAD_VALUE;
-	}
 
 	ssize_t outLength = 0;