MM-46503: Escape incorrect pg user search query (#20863)

There were 2 main problems after https://github.com/mattermost/mattermost-server/pull/20367.

1. The : wasn't escaped.
2. Empty search didn't work.

For 1, we escape the ':'. For 2, we just use
pattern search.

```release-note
NONE
```

store/storetest/user_store.go

@@ -2800,6 +2800,22 @@ func testUserStoreSearch(t *testing.T, ss store.Store) {
 			},
 			[]*model.User{u3},
 		},
+		{
+			"escape :",
+			t1id,
+			"ji:m",
+			&model.UserSearchOptions{},
+			[]*model.User{},
+		},
+		{
+			"wildcard search",
+			t1id,
+			"@",
+			&model.UserSearchOptions{
+				Limit: model.UserSearchDefaultLimit,
+			},
+			[]*model.User{u2, u1, u3},
+		},
 	}
 
 	for _, testCase := range testCases {