Internal: replace empty index block checks with global block checks

[tlrx]

While working on #9203 we noticed (see comment) that many checkBlock() methods were checking blocks on an empty index like this:

protected ClusterBlockException checkBlock(PutIndexTemplateRequest request, ClusterState state) {
        return state.blocks().indexBlockedException(ClusterBlockLevel.METADATA, "");
}

I think most of this kind of calls can be replaced by a global block check like:

return state.blocks().globalBlockedException(ClusterBlockLevel.METADATA);

[javanna]

+1 we might have to dig if that is indeed a 1:1 replacement, but for sure the different checkBlock variants could be clarified a bit. I think if it's about index blocks they shouldn't accept empty indices ideally.