do not return NULL, if database is empty

Null causes trouble during further processing, i.e. PHP warnings in the
API processing and later on problems building the dashboard widget in
Javascript, because all methods expect the array to have all keys.

Move the caching (transient) directive into the conditional, so we still
do not cache empty results and so not return NULL anymore.

inc/class-statify-dashboard.php

@@ -283,16 +283,14 @@ public static function get_stats( $force_refresh = false ) {
 		// Prepare data.
 		if ( ! empty( $data['visits'] ) ) {
 			$data['visits'] = array_reverse( $data['visits'] );
-		} else {
-			$data = null;
-		}
 
-		// Make cache.
-		set_transient(
-			'statify_data',
-			$data,
-			MINUTE_IN_SECONDS * 15
-		);
+			// Make cache.
+			set_transient(
+				'statify_data',
+				$data,
+				MINUTE_IN_SECONDS * 15
+			);
+		}
 
 		return $data;
 	}

tests/test-dashboard.php

@@ -138,7 +138,15 @@ function ( $original ) use ( &$original_capture, &$override ) {
 	 */
 	public function test_get_stats() {
 		// Initially the database is empty.
-		$this->assertNull( $this->get_stats(), 'Expected NULL stats for empty database' );
+		$this->assertEquals(
+			array(
+				'referrer' => array(),
+				'target'   => array(),
+				'visits'   => array(),
+			),
+			$this->get_stats(),
+			'Expected empty dataset for empty database'
+		);
 
 		// Now insert data for the last 3 days.
 		$date1 = new DateTime();