Merge pull request 'Improve check for non-existent variables' (#2) from custom-field-check-with-isset into main

Reviewed-on: #2
Reviewed-by: teutat3s <teutat3s@noreply.git.pub.solar>
Reviewed-by: b12f <b12f@noreply.git.pub.solar>

momo.php

@@ -229,7 +229,7 @@ footer .notes {
 
   <p class="invoice-meta">
     Rechnungsnummer: <?php echo $invoice->invoice_number; ?><br />
-    <?php if ($custom_fields['invoice']['Leistungszeitraum-Anfang'] && $custom_fields['invoice']['Leistungszeitraum-Ende']): ?>
+    <?php if (isset($custom_fields['invoice']['Leistungszeitraum-Anfang']) && isset($custom_fields['invoice']['Leistungszeitraum-Ende'])): ?>
       Leistungszeitraum:
       <?php echo date_from_mysql($custom_fields['invoice']['Leistungszeitraum-Anfang'], true); ?>
       -
@@ -237,7 +237,7 @@ footer .notes {
     <?php endif; ?>
   </p>
 
-  <?php if ($custom_fields['invoice']['Beschreibung']): ?>
+  <?php if (isset($custom_fields['invoice']['Beschreibung'])): ?>
     <p>
       <?php echo $custom_fields['invoice']['Beschreibung']; ?>
     </p>