fix(rules): `no-dup-id` handles when id is set but omitted value

src/rules/no-dup-id.spec.ts

@@ -17,6 +17,11 @@ describe("rule no-dup-id", () => {
 		expect(report).toBeValid();
 	});
 
+	it("should not report when id is missing value", () => {
+		const report = htmlvalidate.validateString("<hr id><hr id>");
+		expect(report).toBeValid();
+	});
+
 	it("should report when id is duplicated", () => {
 		const report = htmlvalidate.validateString(
 			'<p id="foo"></p><p id="foo"></p>'

src/rules/no-dup-id.ts

@@ -13,13 +13,19 @@ class NoDupID extends Rule {
 		this.on("dom:ready", (event: DOMReadyEvent) => {
 			const existing: { [key: string]: boolean } = {};
 			const elements = event.document.querySelectorAll("[id]");
-			elements.forEach(el => {
+			for (const el of elements) {
+				/* handle when the id attribute is set but omitted value: <p id></p> */
+				if (!el.id) {
+					continue;
+				}
+
 				if (el.id in existing) {
 					const attr = el.getAttribute("id");
 					this.report(el, `Duplicate ID "${el.id}"`, attr.valueLocation);
 				}
+
 				existing[el.id] = true;
-			});
+			}
 		});
 	}
 }