Change expectation for invalid/missing shadowRootMode IDL to null

Per the spec discussion:

whatwg/html#5465 (comment)

this changes the expectation for the tests to be:

document.createElement("template").shadowRootMode === ""

rather than === null.

Note that this CL does not change Chromium's behavior, and as such
I cannot check if there are other tests that implicitly also
do a similar check. More tests might need to change once I (or
someone) implements this new behavior in a browser.

Bug: 1379513
Change-Id: I0ea1371d612d524fdecc287f3f1790d601c64982

shadow-dom/declarative/declarative-shadow-dom-basic.html

@@ -39,9 +39,9 @@
   t.setAttribute('shadowrootmode','OpEn');
   assert_equals(t.shadowRootMode,'open','Case insensitive');
   t.setAttribute('shadowrootmode','INVALID');
-  assert_equals(t.shadowRootMode,null,'Invalid values map to null');
+  assert_equals(t.shadowRootMode,'','Invalid values map to empty string');
   t.removeAttribute('shadowrootmode');
-  assert_equals(t.shadowRootMode,null,'No shadowrootmode attribute maps to null');
+  assert_equals(t.shadowRootMode,'','No shadowrootmode attribute maps to empty string');
 }, 'Shadowrootmode reflection');
 
 test(() => {