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/sqlstore/user_store.go

@@ -1502,10 +1502,21 @@ func generateSearchQuery(query sq.SelectBuilder, terms []string, fields []string
 		termArgs := []any{}
 		var dbSpecificTerm string
 
+		if isPostgreSQL {
+			// Escaping the : in case of a Postgres search.
+			term = strings.ReplaceAll(term, ":", "\\:")
+		}
+
 		for _, field := range fields {
 			if isPostgreSQL {
-				searchFields = append(searchFields, fmt.Sprintf("to_tsvector(lower(%[1]s)) @@ to_tsquery(concat(lower(?),':*'))", field))
-				dbSpecificTerm = strings.TrimLeft(term, "@")
+				if strings.TrimLeft(term, "@") == "" {
+					// For wildcard search, we need to fall back to pattern matching.
+					searchFields = append(searchFields, fmt.Sprintf("%s ILIKE ? escape '*' ", field))
+					dbSpecificTerm = fmt.Sprintf("%s%%", strings.TrimLeft(term, "@"))
+				} else {
+					searchFields = append(searchFields, fmt.Sprintf("to_tsvector(lower(%[1]s)) @@ to_tsquery(concat(lower(?),':*'))", field))
+					dbSpecificTerm = strings.TrimLeft(term, "@")
+				}
 			} else {
 				searchFields = append(searchFields, fmt.Sprintf("%s LIKE ? escape '*' ", field))
 				dbSpecificTerm = fmt.Sprintf("%s%%", strings.TrimLeft(term, "@"))