fix(systemtags): handle null dav value

Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

apps/dav/lib/SystemTag/SystemTagPlugin.php

@@ -443,7 +443,11 @@ class SystemTagPlugin extends \Sabre\DAV\ServerPlugin {
 			}
 
 			if (isset($props[self::COLOR_PROPERTYNAME])) {
-				$color = $props[self::COLOR_PROPERTYNAME];
+				$propValue = $props[self::COLOR_PROPERTYNAME];
+				if ($propValue === '' || $propValue === 'null') {
+					$propValue = null;
+				}
+				$color = $propValue;
 				$updateTag = true;
 			}
 

apps/systemtags/src/services/api.ts

@@ -107,7 +107,7 @@ export const updateTag = async (tag: TagWithId): Promise<void> => {
 				<oc:display-name>${tag.displayName}</oc:display-name>
 				<oc:user-visible>${tag.userVisible}</oc:user-visible>
 				<oc:user-assignable>${tag.userAssignable}</oc:user-assignable>
-				<nc:color>${tag.color}</nc:color>
+				<nc:color>${tag?.color || null}</nc:color>
 			</d:prop>
 		</d:set>
 	</d:propertyupdate>`