[FileCheck] Fix parsing empty global and pseudo variable names (#83667)

Reland #82595 with fixes of build failures related to colored output.
See https://lab.llvm.org/buildbot/#/builders/139/builds/60549
Use `%ProtectFileCheckOutput` to avoid colored output.
Original commit message below.

In `Pattern::parseVariable`, for global variables (those starting with '$')
and for pseudo variables (those starting with '@') the first character is
consumed before actual variable name parsing. If the name is empty, it
leads to out-of-bound access to the corresponding `StringRef`.

This patch adds an if statement against the case described.