Creating a std::string with a null pointer is undefined behaviour.

- cppreference mentions this about the constructor that receives a
  const char *:
  - Constructs the string with the contents initialized with a copy of
    the null-terminated character string pointed to by s. The length of
    the string is determined by the first null character. The behavior
    is undefined if [s, s + Traits::length(s)) is not a valid range
    (for example, if s is a null pointer).
- C++23 introduces a deleted constructor to prevent this in static
  scenarios, which is how this issue was detected.

src/actions/transformations/url_encode.cc

@@ -48,7 +48,7 @@ std::string UrlEncode::url_enc(const char *input,
     len = input_len * 3 + 1;
     d = rval = reinterpret_cast<char *>(malloc(len));
     if (rval == NULL) {
-        return NULL;
+        return {};
     }
 
     /* ENH Only encode the characters that really need to be encoded. */