Bail out if no fonts to process.

1. Adds guard clause to WP_Font_Face::generate_and_print().
2. Adds unhappy test for it.
3. Adds similar unhappy test for wp_print_font_faces().

lib/experimental/fonts/class-wp-font-face.php

@@ -118,6 +118,11 @@ function_exists( 'current_theme_supports' ) && ! current_theme_supports( 'html5'
 	public function generate_and_print( array $fonts ) {
 		$fonts = $this->validate_fonts( $fonts );
 
+		// Bail out if there are no fonts are given to process.
+		if ( empty( $fonts ) ) {
+			return;
+		}
+
 		printf(
 			$this->get_style_element(),
 			$this->get_css( $fonts )

phpunit/fonts/wpFontFace/generateAndPrint-test.php

@@ -32,6 +32,14 @@
 class Tests_Fonts_WPFontFace_GenerateAndPrint extends WP_UnitTestCase {
 	use WP_Font_Face_Tests_Datasets;
 
+	public function test_should_not_generate_and_print_when_no_fonts() {
+		$font_face = new WP_Font_Face();
+		$fonts     = array();
+
+		$this->expectOutputString( '' );
+		$font_face->generate_and_print( $fonts );
+	}
+
 	/**
 	 * @dataProvider data_should_print_given_fonts
 	 *

phpunit/fonts/wpPrintFontFaces-test.php

@@ -38,6 +38,13 @@ public static function set_up_before_class() {
 		parent::set_up_before_class();
 	}
 
+	public function test_should_not_print_when_no_fonts() {
+		switch_theme( 'block-theme' );
+
+		$this->expectOutputString( '' );
+		wp_print_font_faces();
+	}
+
 	/**
 	 * @dataProvider data_should_print_given_fonts
 	 *