urlapi: add CURLU_GET_EMPTY for empty queries and fragments

By default the API inhibits empty queries and fragments extracted.
Unless this new flag is set.

This also makes the behavior more consistent: without it set, zero
length queries and fragments are considered not present in the URL. With
the flag set, they are returned as a zero length strings if they were in
fact present in the URL.

This applies when extracting the individual query and fragment
components and for the full URL.

Closes #13396

include/curl/urlapi.h

@@ -99,6 +99,9 @@ typedef enum {
 #define CURLU_ALLOW_SPACE (1<<11)       /* Allow spaces in the URL */
 #define CURLU_PUNYCODE (1<<12)          /* get the host name in punycode */
 #define CURLU_PUNY2IDN (1<<13)          /* punycode => IDN conversion */
+#define CURLU_GET_EMPTY (1<<14)         /* allow empty queries and fragments
+                                           when extracting the URL or the
+                                           components */
 
 typedef struct Curl_URL CURLU;