Small code and doc improvements

Remove unnecessary else-branch and use Collections.isEmpty().
Add doc comment for splitFilename()

app/src/main/java/org/schabi/newpipe/streams/io/StoredDirectoryHelper.java

@@ -102,9 +102,10 @@ public class StoredDirectoryHelper {
             }
         }
 
-        if (matches.size() < 1) {
+        if (matches.isEmpty()) {
             return createFile(name, mime, true);
-        } else {
+        }
+
         // check if the filename is in use
         String lcName = name.toLowerCase();
         for (final String testName : matches) {
@@ -114,11 +115,10 @@ public class StoredDirectoryHelper {
             }
         }
 
-            // check if not in use
+        // create file if filename not in use
         if (lcName != null) {
             return createFile(name, mime, true);
         }
-        }
 
         Collections.sort(matches, String::compareTo);
 
@@ -261,6 +261,12 @@ public class StoredDirectoryHelper {
         }
     }
 
+    /**
+     * Splits the filename into the name and extension.
+     *
+     * @param filename The filename to split
+     * @return A String array with the name at index 0 and extension at index 1
+     */
     private static String[] splitFilename(@NonNull final String filename) {
         final int dotIndex = filename.lastIndexOf('.');