Fixed problem with matching against elements with no children

src/index.js

@@ -1625,8 +1625,10 @@ module.exports = {
     expect.exportAssertion(
       '<DOMDocument|DOMElement|DOMDocumentFragment|DOMNodeList> [not] to contain <DOMElement|object|string>',
       (expect, subject, value) => {
-        const nodes = subject.childNodes || subject;
-        const isHtml = isInsideHtmlDocument(nodes[0]);
+        const nodes = subject.childNodes || makeAttachedDOMNodeList(subject);
+        const isHtml = isInsideHtmlDocument(
+          subject.childNodes ? subject : nodes
+        );
         const valueType = expect.findTypeOf(value);
         let spec = value;
 

test/index.spec.js

@@ -3167,6 +3167,20 @@ describe('unexpected-dom', () => {
       );
     });
 
+    it('fails when matching against an element with no children', () => {
+      expect(
+        () => {
+          expect(
+            parseHtmlNode('<div></div>'),
+            'to contain',
+            '<span>Jane Doe</span>'
+          );
+        },
+        'to throw',
+        'expected <div></div> to contain <span>Jane Doe</span>'
+      );
+    });
+
     it('should not match directly on the subject', () => {
       expect(
         () => {
@@ -3351,6 +3365,14 @@ describe('unexpected-dom', () => {
       );
     });
 
+    it('succeeds if the element has no children', () => {
+      expect(
+        parseHtmlNode('<div></div>'),
+        'not to contain',
+        '<span>Jane Doe</span>'
+      );
+    });
+
     it('shows a diff if the given structure is present', () => {
       expect(
         () => {