Fix inconsistency in getQuantity and add docs

`getQuantity()` was being called in a couple of places with `zeroCaseStringId=0`, but that wasn't documented anywhere, and if `count==0` then `getString(zeroCaseStringId /* == 0 */)` would be returned which doesn't make sense

app/src/main/java/org/schabi/newpipe/util/Localization.java

@@ -426,12 +426,24 @@ public final class Localization {
         return new BigDecimal(value).setScale(scale, RoundingMode.HALF_UP).doubleValue();
     }
 
+    /**
+     * A wrapper around {@code context.getResources().getQuantityString()} with some safeguard.
+     *
+     * @param context the Android context
+     * @param pluralId the ID of the plural resource
+     * @param zeroCaseStringId the resource ID of the string to use in case {@code count=0},
+     *                         or 0 if the plural resource should be used in the zero case too
+     * @param count the number that should be used to pick the correct plural form
+     * @param formattedCount the formatting parameter to substitute inside the plural resource,
+     *                       ideally just {@code count} converted to string
+     * @return the formatted string with the correct pluralization
+     */
     private static String getQuantity(@NonNull final Context context,
                                       @PluralsRes final int pluralId,
                                       @StringRes final int zeroCaseStringId,
                                       final long count,
                                       final String formattedCount) {
-        if (count == 0) {
+        if (count == 0 && zeroCaseStringId != 0) {
             return context.getString(zeroCaseStringId);
         }