escapeString() can get a null value.

lib/Serendipity/Database/DbAbstract.php

@@ -46,7 +46,7 @@ abstract class DbAbstract
     /**
      * Returns an escaped string, so that it can be safely included in a SQL string encapsulated within quotes, without allowing SQL injection.
      */
-    public function escapeString(string $string): string
+    public function escapeString($string): string
     {
         return $string;
     }

lib/Serendipity/Database/MysqliDatabase.php

@@ -200,7 +200,7 @@ class MysqliDatabase extends DbAbstract
     /**
      * Returns an escaped string, so that it can be safely included in a SQL string encapsulated within quotes, without allowing SQL injection.
      */
-    public function escapeString(string $string): string
+    public function escapeString($string): string
     {
         return mysqli_escape_string($this->db_conn, $string);
     }

lib/Serendipity/Database/PdoPostgresDatabase.php

@@ -84,7 +84,7 @@ class PdoPostgresDatabase extends DbAbstract
     /**
      * Returns an escaped string, so that it can be safely included in a SQL string encapsulated within quotes, without allowing SQL injection.
      */
-    public function escapeString(string $string): string
+    public function escapeString($string): string
     {
         return substr($this->db_conn->quote($string), 1, -1);
     }

lib/Serendipity/Database/PdoSqliteDatabase.php

@@ -120,7 +120,7 @@ class PdoSqliteDatabase extends DbAbstract
     /**
      * Returns an escaped string, so that it can be safely included in a SQL string encapsulated within quotes, without allowing SQL injection.
      */
-    public function escapeString(string $string): string
+    public function escapeString($string): string
     {
         return substr($this->db_conn->quote($string), 1, -1);
     }

lib/Serendipity/Database/PostgresDatabase.php

@@ -89,7 +89,7 @@ class PostgresDatabase extends DbAbstract
     /**
      * Returns an escaped string, so that it can be safely included in a SQL string encapsulated within quotes, without allowing SQL injection.
      */
-    public function escapeString(string $string): string
+    public function escapeString($string): string
     {
         return pg_escape_string($string);
     }

lib/Serendipity/Database/SqlRelayDatabase.php

@@ -322,7 +322,7 @@ class SqlRelayDatabase extends DbAbstract
     /**
      * Returns an escaped string, so that it can be safely included in a SQL string encapsulated within quotes, without allowing SQL injection.
      */
-    public function escapeString(string $string): string
+    public function escapeString($string): string
     {
         static $search  = array("\x00", '%',   "'",   '\"');
         static $replace = array('%00',  '%25', "''", '\\\"');

lib/Serendipity/Database/Sqlite3Database.php

@@ -60,7 +60,7 @@ class Sqlite3Database extends DbAbstract
     /**
      * Returns an escaped string, so that it can be safely included in a SQL string encapsulated within quotes, without allowing SQL injection.
      */
-    public function escapeString(string $string): string
+    public function escapeString($string): string
     {
         static $search  = array("\x00", '%',   "'",   '\"');
         static $replace = array('%00',  '%25', "''", '\\\"');

lib/Serendipity/Database/SqliteDatabase.php

@@ -64,7 +64,7 @@ class SqliteDatabase extends DbAbstract
     /**
      * Returns an escaped string, so that it can be safely included in a SQL string encapsulated within quotes, without allowing SQL injection.
      */
-    public function escapeString(string $string): string
+    public function escapeString($string): string
     {
         static $search  = array("\x00", '%',   "'",   '\"');
         static $replace = array('%00',  '%25', "''", '\\\"');