Avoid pass NULL to memcmp() (#3844)

If we exec "StringVal(len=0, ptr="") == StringVal(len=0,ptr=NULL)", it will pass NULL ptr to memcmp(). It should be avoided.

be/src/udf/udf.h

@@ -644,7 +644,7 @@ struct StringVal : public AnyVal {
             return false;
         }
 
-        return ptr == other.ptr || memcmp(ptr, other.ptr, len) == 0;
+        return len == 0 || ptr == other.ptr || memcmp(ptr, other.ptr, len) == 0;
     }
 
     bool operator!=(const StringVal& other) const {