fix(web): fix TPM check when using the legacy AutoYaST mode

[imobachgs]

Problem

When using the legacy AutoYaST mode, the web UI crashes at the end of the installation when trying
to find our whether it is using TPM or not.

• Unexpected application error #1918

Solution

Properly handle that case:

• fetchConfig now returns null instead of undefined. According to TanStack Query
  documentation, queryFn
  Must return a promise that will either resolve data or throw an error. The data cannot be
  undefined.
• usingTpm function now handles the case where the configuration is null.

Testing

• Tested manually

[joseivanlopez]

LGTM

[teclator]

LGTM

[dgdavid]

Just a note.

Would be nice to be consistent in the way we return early from a component to tell React renders nothing.

I for sure know that no matters if you return undefined or null, React will understand both of them as nothing. But we used to go for

if (something) return;

Which returns undefined, the default return value for a JavaScript function.

Now we've a small dilemma with it comes to consistence because, as stated in the PR, react-query requires the query returns always something (null meaning nothing).

We can, of course, choose a default return value for everything or one for components and another for queries, but betting for consistency in each case for easing code maintenance.