transformations: Fix edge-cases with {param.value} and {uri.param}

For inputs containing only the parameter part, these transformations
would return a bogus {NULL, 0} value (flagged as PV_VAL_STR), which
cannot be used in assignments or conditional checks:

Credits to Bogdan-Andrei Iancu for finding this issue!

str.h

@@ -69,6 +69,8 @@ typedef struct __str_const str_const;
 /* str initialization */
 #define STR_NULL ((str){NULL, 0})
 #define STR_NULL_const ((str_const){NULL, 0})
+#define STR_EMPTY ((str){"", 0})
+#define STR_EMPTY_const ((str_const){"", 0})
 #define str_init(_string)  ((str){_string, sizeof(_string) - 1})
 #define str_const_init(_string)  ((str_const){_string, sizeof(_string) - 1})
 

transformations.c

@@ -1204,7 +1204,10 @@ int tr_eval_uri(struct sip_msg *msg, tr_param_t *tp, int subtype,
 				if (pit->name.len==sv.len
 						&& strncasecmp(pit->name.s, sv.s, sv.len)==0)
 				{
-					val->rs = pit->body;
+					if (ZSTR(pit->body))
+						val->rs = STR_EMPTY;
+					else
+						val->rs = pit->body;
 					goto done;
 				}
 			}
@@ -2226,7 +2229,10 @@ int tr_eval_paramlist(struct sip_msg *msg, tr_param_t *tp, int subtype,
 				if (pit->name.len==sv.len
 						&& strncasecmp(pit->name.s, sv.s, sv.len)==0)
 				{
-					val->rs = pit->body;
+					if (ZSTR(pit->body))
+						val->rs = STR_EMPTY;
+					else
+						val->rs = pit->body;
 					goto done;
 				}
 			}