Fix Schema Validation for WordPress AMP Plugin where posts don't have a featured image.

class-amp-post-template.php

# Edit AMP Plugin file: amp/includes/class-amp-post-template.php

/*
Change this bit of code to the below uncommented code:

	$image_metadata = $this->get_post_image_metadata();
		if ( $image_metadata ) {
			$metadata['image'] = $image_metadata;
		}

		$this->add_data_by_key( 'metadata', apply_filters( 'amp_post_template_metadata', $metadata, $this->post ) );
	}
*/

		$image_metadata = $this->get_post_image_metadata();
		if ( $image_metadata ) {
			$metadata['image'] = $image_metadata;
		}

		if(empty($metadata['image'])){
			$metadata['image'] = array(
				'@type' => 'ImageObject',
				'url' => 'URL OF IMAGE',
				'height' => 300,
				'width' => 700,
			);
		}

		$this->add_data_by_key( 'metadata', apply_filters( 'amp_post_template_metadata', $metadata, $this->post ) );
	}