Fix compatible with PHP 8.1 (#17)

* Fix compatible with PHP 8.1

There was an error in PHP 8.1: "...with message 'strtolower(): Passing null to parameter #1 ($string) of type string is deprecated..."
I fixed it.

* Added php 8.1 to unit tests

* Also adding workflow_dispatch to tigger gha

Co-authored-by: Aaron de Mello <314152+AaronDDM@users.noreply.github.com>
Co-authored-by: AaronDDM <aaron.d@nylas.com>

.github/workflows/unit-test.yaml

@@ -1,14 +1,14 @@
 name: unit-test
 
-on: [push]
+on: [workflow_dispatch,push]
 
 jobs:
   run:    
     runs-on: ${{ matrix.operating-system }}
     strategy:      
       matrix:
         operating-system: [ubuntu-latest]
-        php-versions: ['7.2', '7.3', '7.4', '8.0']
+        php-versions: ['7.2', '7.3', '7.4', '8.0', '8.1']
     name: PHP v${{ matrix.php-versions }}
     steps:
     - name: Checkout

lib/Writer/XMLWriterService.php

@@ -86,7 +86,7 @@ protected function buildXML($array, \XMLWriter &$xmlWriter)
                 $this->buildXML($element['value'], $xmlWriter);
             } else {
                 if ($element['value'] !== null) {
-                    switch (strtolower($element['type'])) {
+                    switch (strtolower($element['type'] ?? '')) {
                         case 'cdata':
                             $xmlWriter->writeCdata($element['value']);
                             break;