Add uri null handling into isScopedToUri logic

In cases like: https://hypothes.is/a/rGSnNwCKSuiG3VT__k3VvA where the
sidebar is not being used, the document uri is set to null in the groups
service and passed to the combineGroups function. This means that the
isScopedToUri logic must handle the case where uri=null.

src/sidebar/util/test/groups-test.js

@@ -129,6 +129,12 @@ describe('sidebar.util.groups', () => {
         isScopedToUri: false,
         uri: 'https://foo.com/bar',
       },
+      {
+        description: 'sets `isScopedToUri` to true if `uri` is null',
+        scopes: { enforced: true, uri_patterns: ['http://foo.com*'] },
+        isScopedToUri: true,
+        uri: null,
+      },
       {
         description: 'it permits multiple *s in the scopes uri pattern',
         scopes: { enforced: true, uri_patterns: ['https://foo.com*bar*'] },