MDL-37742 add debugging message when $DB->sql_empty() used

lib/dml/moodle_database.php

@@ -2019,6 +2019,7 @@ abstract class moodle_database {
      * @return string An empty string.
      */
     function sql_empty() {
+        debugging("sql_empty() is deprecated, please use empty string '' as sql parameter value instead", DEBUG_DEVELOPER);
         return '';
     }
 

lib/dml/oci_native_moodle_database.php

@@ -1605,17 +1605,6 @@ class oci_native_moodle_database extends moodle_database {
         }
     }
 
-    /**
-     * This used to return empty string replacement character.
-     *
-     * @deprecated use bound parameter with empty string instead
-     *
-     * @return string An empty string.
-     */
-    public function sql_empty() {
-        return '';
-    }
-
     public function sql_order_by_text($fieldname, $numchars=32) {
         return 'dbms_lob.substr(' . $fieldname . ', ' . $numchars . ',1)';
     }

lib/dml/tests/dml_test.php

@@ -3927,6 +3927,7 @@ class dml_testcase extends database_driver_testcase {
         $tablename = $table->getName();
 
         $this->assertSame('', $DB->sql_empty()); // Since 2.5 the hack is applied automatically to all bound params.
+        $this->assertDebuggingCalled();
 
         $table->add_field('id', XMLDB_TYPE_INTEGER, '10', XMLDB_UNSIGNED, XMLDB_NOTNULL, XMLDB_SEQUENCE, null);
         $table->add_field('name', XMLDB_TYPE_CHAR, '255', null, null, null, null);