Refactor agtype_access_operator

Refactored the agtype_access_operator and support functions to
eliminate the constant conversions back-and-forth.

Also corrected the response to AGTV_NULL

Moved get_agtype_key from age_vle.c to agtype.c for usage by
others.

Adjusted 2 regression tests.

regress/expected/expr.out

@@ -5404,7 +5404,7 @@ SELECT * FROM cypher('case_statement', $$
 $$ ) AS (i agtype, j agtype, case_statement agtype);
   i   |     j     | case_statement 
 ------+-----------+----------------
- 1    | null      | "i is 1"
+ 1    |           | "i is 1"
  "a"  | "b"       | "j is b"
  0    | 1         | "0 AND 1"
  true | false     | "i or j true"
@@ -5427,7 +5427,7 @@ SELECT * FROM cypher('case_statement', $$
 $$ ) AS (j agtype, case_statement agtype);
      j     | case_statement 
 -----------+----------------
- null      | "not a or b"
+           | "not a or b"
  "b"       | "b"
  1         | 1
  false     | false

src/backend/utils/adt/age_vle.c

@@ -101,8 +101,6 @@ typedef struct VLE_path_container
 
 /* declarations */
 /* agtype functions */
-static agtype_value *get_agtype_key(agtype_value *agtv, char *search_key,
-                                    int search_key_len);
 static agtype_iterator *get_next_object_pair(agtype_iterator *it,
                                              agtype_container *agtc,
                                              agtype_value *key,
@@ -387,42 +385,6 @@ static bool is_an_edge_match(VLE_local_context *vlelctx, edge_entry *ee)
     return true;
 }
 
-/*
- * Helper function to iterate through all object pairs, looking for a specific
- * key. It will return the key or NULL if not found.
- */
-static agtype_value *get_agtype_key(agtype_value *agtv, char *search_key,
-                                    int search_key_len)
-{
-    int i = 0;
-
-    if (agtv == NULL || search_key == NULL || search_key_len <= 0)
-    {
-        return NULL;
-    }
-
-    /* iterate through all pairs */
-    for (i = 0; i < agtv->val.object.num_pairs; i++)
-    {
-        agtype_value *agtv_key = &agtv->val.object.pairs[i].key;
-        agtype_value *agtv_value = &agtv->val.object.pairs[i].value;
-
-        char *current_key = agtv_key->val.string.val;
-        int current_key_len = agtv_key->val.string.len;
-
-        Assert(agtv_key->type == AGTV_STRING);
-
-        /* check for an id of type integer */
-        if (current_key_len == search_key_len &&
-            pg_strcasecmp(current_key, search_key) == 0)
-        {
-            return agtv_value;
-        }
-    }
-
-    return NULL;
-}
-
 /*
  * Helper function to free up the memory used by the VLE_local_context.
  *