fix: String_IsNull/String_Offset helpers + remove pointer-to-int casts

- Add String_IsNull and String_Offset wrappers in lib/String.bux
- Add bux_str_is_null and bux_str_offset runtime functions in rt/runtime.c
- Update String_Replace to use String_IsNull instead of String_Len(pos)==0
- Replace pointer-to-uint casts (as uint == 0, pointer arithmetic) across apps/
- Parser newline skipping fixes in src/parser.bux

apps/jwt-pitbul/src/Main.bux

@@ -96,7 +96,7 @@ func ResolveKey(alg: JwtAlg, keyArg: String) -> String {
        alg.tag == JwtAlg_ES256 || alg.tag == JwtAlg_ES384 {
         if bux_file_exists(keyArg) != 0 {
             let pem: String = bux_read_file(keyArg);
-            if pem as uint != 0 && String_Len(pem) > 0 {
+            if String_Len(pem) > 0 && String_Len(pem) > 0 {
                 return pem;
             }
             Print("ERROR: could not read PEM file: ");