Fix Utils.nonEmptyAndNullJoin

When using the index here, it the index needs to be decremented once an element is removed. To cirecumvent this, the native Collections.removeIf() method is used.

extractor/src/test/java/org/schabi/newpipe/extractor/utils/UtilsTest.java

@@ -20,6 +20,7 @@ public class UtilsTest {
     @Test
     public void testJoin() {
         assertEquals("some,random,stuff", Utils.join(",", Arrays.asList("some", "random", "stuff")));
+        assertEquals("some,random,not-null,stuff", Utils.nonEmptyAndNullJoin(",", new String[]{"some", "null", "random", "", "not-null", null, "stuff"}));
     }
 
     @Test