Avoid PHP warnings with PHP >= 7.2.x

Fix PHP Warning:  sizeof(): Parameter must be an array or an object that implements Countable on line 263

includes/admin/post-types/meta-boxes/class-sp-meta-box-calendar-data.php

@@ -39,8 +39,11 @@ class SP_Meta_Box_Calendar_Data {
 		$title_format = get_option( 'sportspress_event_list_title_format', 'title' );
 		$time_format = get_option( 'sportspress_event_list_time_format', 'combined' );
 
-		if ( is_array( $usecolumns ) )
+		if ( is_array( $usecolumns ) ) {
 			$usecolumns = array_filter( $usecolumns );
+		}else{
+			$usecolumns = array();
+		}
 		?>
 		<div class="sp-data-table-container">
 			<table class="widefat sp-data-table sp-calendar-table">
@@ -272,4 +275,4 @@ class SP_Meta_Box_Calendar_Data {
 		</div>
 		<?php
 	}
-}
+}