fix img-reduntant-alt bug (#4771)

src/compiler/compile/nodes/Element.ts

@@ -488,7 +488,7 @@ export default class Element extends Node {
 			if (alt_attribute && !aria_hidden_exist) {
 				const alt_value = alt_attribute.get_static_value();
 
-				if (alt_value.match(/\b(image|picture|photo)\b/i)) {
+				if (alt_value && alt_value.match(/\b(image|picture|photo)\b/i)) {
 					component.warn(this, {
 						code: `a11y-img-redundant-alt`,
 						message: `A11y: Screenreaders already announce <img> elements as an image.`

test/validator/samples/a11y-img-redundant-alt/input.svelte

@@ -5,3 +5,4 @@
 <img src="foo" alt="Picture of baz fixing a bug." />
 <img src="bar" alt="Plant doing photosynthesis in the afternoon" />
 <img src="foo" alt="Picturesque food" />
+<img src="baz" alt={'baz'} />