don't use strings for store empty custom views

You save empty view like @"" strings. 0_o

    - (void)setCustomView:(UIView *)view forState:(SVPullToRefreshState)state {
        id viewPlaceholder = view;
        
        if(!viewPlaceholder)
            viewPlaceholder = @"";
        
        if(state == SVPullToRefreshStateAll)
            [self.viewForState replaceObjectsInRange:NSMakeRange(0, 3) withObjectsFromArray:@[viewPlaceholder, viewPlaceholder, viewPlaceholder]];
        else
            [self.viewForState replaceObjectAtIndex:state withObject:viewPlaceholder];
        
        [self setNeedsLayout];
    }

It's look copy-paste from 

    - (void)setSubtitle:(NSString *)subtitle forState:(SVPullToRefreshState)state {
        if(!subtitle)
            subtitle = @"";
        
        if(state == SVPullToRefreshStateAll)
            [self.subtitles replaceObjectsInRange:NSMakeRange(0, 3) withObjectsFromArray:@[subtitle, subtitle, subtitle]];
        else
            [self.subtitles replaceObjectAtIndex:state withObject:subtitle];
        
        [self setNeedsLayout];
    }