Bug 1795801 - Avoid logspam due to xpcshell tests without any system addon set throwing from cleanupStatingDir. r=willdurand

Not actually related to the change that the bug is tracking, but I did notice this an unexpected error
that was spamming the logs while I was investigating failures from one of the event pages / persistent
listeners tests as part of the rework of the persistent listeners data written in the XPIStates JSONFile.

Depends on D171769

Differential Revision: https://phabricator.services.mozilla.com/D171770

toolkit/mozapps/extensions/internal/XPIInstall.jsm

@@ -3231,6 +3231,12 @@ class DirectoryInstaller {
   cleanStagingDir(aLeafNames = []) {
     let dir = this.getStagingDir();
 
+    // SystemAddonInstaller getStatingDir may return null if there isn't
+    // any addon set directory returned by SystemAddonInstaller._loadAddonSet.
+    if (!dir) {
+      return;
+    }
+
     for (let name of aLeafNames) {
       let file = getFile(name, dir);
       recursiveRemove(file);