Use asterisk instead of empty string to clear all cached entries (#63907) (#63989)

The officially supported way to clearing all entries from a cache is to use 
wildcard of either * or _all. Though empty string has the same effect, it was
never intended. Therefore the tests should not use empty string and this PR
changes them to use *.

x-pack/plugin/src/test/resources/rest-api-spec/test/api_key/10_basic.yml

@@ -195,7 +195,7 @@ teardown:
 
   - do:
       security.clear_api_key_cache:
-        ids: ""
+        ids: "*"
 
   - match: { _nodes.failed: 0 }
 

x-pack/plugin/src/test/resources/rest-api-spec/test/privileges/10_basic.yml

@@ -338,5 +338,5 @@ teardown:
 
   - do:
       security.clear_cached_privileges:
-        application: ""
+        application: "*"
   - match: { _nodes.failed: 0 }