Fixed #16208 - don't call preg_split() with a null

[uberbrady]

PHP has made the poor decision to start throwing deprecation warnings if you call preg_split() with a null as the second parameter - behavior that has been completely permissible for the last decade or more in PHP.

These somewhat pointless changes should silence those deprecation warnings and allow us to remain compatible with the latest versions of PHP.

[what-the-diff]

PR Summary

• Enhancement in SettingsSamlRequest.php File Handling
  The updates made have enhanced the handling of empty inputs in SettingsSamlRequest.php file. It now provides a default value of an empty string when retrieving saml_custom_settings, thus improving the method's reliability and safety.

• Improvements in CustomField.php File Error Prevention
  Changes in the CustomField.php file have resulted in better error prevention. It now uses a null coalescing operator when processing field_values, which effectively eliminates potential errors that may arise from undefined values.

• Robustness Enhancement in Saml.php File
  The handling of null values in the Saml.php file has been improved. A null coalescing operator is now used when splitting saml_custom_settings, ensuring better robustness of the software against such values.