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
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4954120
Commit-Queue: Mason Freed <masonf@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1213658}

third_party/blink/web_tests/external/wpt/shadow-dom/declarative/declarative-shadow-dom-basic-expected.txt

@@ -0,0 +1,19 @@
+This is a testharness.js-based test.
+[PASS] Declarative Shadow DOM: Basic test
+[PASS] Declarative Shadow DOM: Feature detection
+[FAIL] Shadowrootmode reflection
+  assert_equals: Invalid values map to empty string expected (string) "" but got (object) null
+[PASS] Declarative Shadow DOM: Fragment parser basic test
+[PASS] Declarative Shadow DOM: Invalid shadow root attribute
+[PASS] Declarative Shadow DOM: Closed shadow root attribute
+[PASS] Declarative Shadow DOM: Missing closing tag
+[PASS] Declarative Shadow DOM: delegates focus attribute
+[PASS] Declarative Shadow DOM: Multiple roots
+[PASS] Declarative Shadow DOM: template containing declarative shadow root
+[PASS] Declarative Shadow DOM: template containing (deeply nested) declarative shadow root
+[PASS] Declarative Shadow DOM: template containing a template containing declarative shadow root
+[PASS] Declarative Shadow DOM: template containing declarative shadow root and UA shadow root
+[PASS] Declarative Shadow DOM: template containing closed declarative shadow root and UA shadow root
+[PASS] Declarative Shadow DOM: template root element
+Harness: the test ran to completion.
+

third_party/blink/web_tests/external/wpt/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(() => {