DEV: Fix several type issues with the api docs (#14016)

`nullable` is no longer a valid type, and types also can't be an empty
string, so just bringing a number of issues with types in compliance
with the openapi spec.

spec/requests/api/schemas/json/admin_user_list_response.json

@@ -48,13 +48,13 @@
         "type": "string"
       },
       "last_seen_age": {
-        "type": ""
+        "type": ["number", "null"]
       },
       "last_emailed_age": {
-        "type": ""
+        "type": ["number", "null"]
       },
       "created_at_age": {
-        "type": ""
+        "type": ["number", "null"]
       },
       "trust_level": {
         "type": "integer"

spec/requests/api/schemas/json/admin_user_response.json

@@ -39,13 +39,13 @@
       "type": "string"
     },
     "last_seen_age": {
-      "type": ""
+      "type": ["number", "null"]
     },
     "last_emailed_age": {
-      "type": ""
+      "type": ["number", "null"]
     },
     "created_at_age": {
-      "type": ""
+      "type": ["number", "null"]
     },
     "trust_level": {
       "type": "integer"
@@ -163,7 +163,7 @@
       "type": "integer"
     },
     "bounce_score": {
-      "type": ""
+      "type": ["integer", "null"]
     },
     "reset_bounce_score_after": {
       "type": ["string", "null"]