Check for navigation icons only if set

Configuration/TypoScript/ContentElements/MenuSubpages.typoscript

@@ -18,6 +18,7 @@ tt_content.menu_subpages {
 
         20 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
         20 {
+          if.isTrue.field = t3o_navicon
           references.fieldName = t3o_navicon
           as = navicon
         }

Configuration/TypoScript/Includes/DataProcessingMenu.typoscript

@@ -12,12 +12,14 @@ page {
         dataProcessing {
           10 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
           10 {
+            if.isTrue.field = t3o_navicon
             references.fieldName = t3o_navicon
             as = nav_icon
           }
 
           20 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
           20 {
+            if.isTrue.field = t3o_nav_content_image
             references.fieldName = t3o_nav_content_image
             as = nav_content_image
           }
@@ -40,6 +42,7 @@ page {
 
           30 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
           30 {
+            if.isTrue.field = t3o_metaicon
             references.fieldName = t3o_metaicon
             as = meta_icon
           }