Forbid the empty-string configuration

[laurentlb]

Description of the bug:

In a bazelrc file, I can write:

common -c opt
common:opt -c opt
common: -c opt

In my case, the last line was a typo. I was surprised to see the line was silently ignored, and wasted time because of this. In practice, the line does something if the user passes --config= without a name!

I don't see why someone would want to do that, so I'd suggest this should be an error when parsing bazelrc files.

Which category does this issue belong to?

Core

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

No response

Which operating system are you running Bazel on?

No response

What is the output of bazel info release?

release 7.1.0

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

No response

Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response