micro-optimize FOO-=$$BAR for empty FOO

there is no point in iterating BAR if FOO is (or became) empty.

Change-Id: I86c89bf0ad726a5ab7ead990a27ef7cc32caebbf
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

qmake/library/proitems.cpp

@@ -396,9 +396,12 @@ void ProStringList::removeAll(const char *str)
 
 void ProStringList::removeEach(const ProStringList &value)
 {
-    for (const ProString &str : value)
+    for (const ProString &str : value) {
+        if (isEmpty())
+            break;
         if (!str.isEmpty())
             removeAll(str);
+    }
 }
 
 void ProStringList::removeEmpty()