[FEATURE] Add option "ifEmptyUnsetKey" to JSON cObject (#650)

This TypoScript option works similar to "ifEmptyReturnNull",
however, the option uses the unset() feature to actually remove
the key from the json output instead of setting this to null.

The code checks for "" and an explicit "false" (bool) when using
the cObject = BOOL

Fixes: #649

Classes/ContentObject/JsonContentObject.php

@@ -121,6 +121,9 @@ public function cObjGet(array $setup, string $addKey = ''): array
                 if ((int)($conf['ifEmptyReturnNull'] ?? 0) === 1 && $content[$theKey] === '') {
                     $content[$theKey] = null;
                 }
+                if ((int)($conf['ifEmptyUnsetKey'] ?? 0) === 1 && ($content[$theKey] === '' || $content[$theKey] === false)) {
+                    unset($content[$theKey]);
+                }
                 if (!empty($contentDataProcessing['dataProcessing.'])) {
                     $content[rtrim($theKey, '.')] = $this->processFieldWithDataProcessing($contentDataProcessing);
                 }