FIX: if text is nil we have to set textColor to placeholderColor otherwise placeholder text doesn´t have the right color

I´m using GCPlaceholderTextView in a UITableViewCell, i set the placeholder like this:
```
SettingsTextViewCell *cell = (SettingsTextViewCell *) [tableView dequeueReusableCellWithIdentifier:@"SettingsTextViewCell"];
        if (!cell) {
            cell = (SettingsTextViewCell *) [[[NSBundle mainBundle] loadNibNamed:@"SettingsTextViewCell"
                                                                           owner:self
                                                                         options:nil] objectAtIndex:0];
        }
cell.textView.placeholder = @"placeholder";
.....
```
and the placeholder color was black. This addition fix the problem with the placeholder color.