project.el: Fix in submodules with nonexistent git root (bug#80402)

project-try-vc--search in a submodule directory tries to find the git
root directory, so the local variable root would be set to nil if it
didn't exist. Calling project--vc-merge-submodules-p on nil would lead
to a crash, so now the while loop aborts beforehand.

lisp/progmodes/project.el

@@ -698,6 +698,7 @@ in the `project-current' call, the timeout is determined by
            vc-handled-backends))
          project)
     (while (and
+            root
             (eq backend 'Git)
             (project--vc-merge-submodules-p root)
             (project--submodule-p root))