BaseTools/Tests: Use quotes around PYTHON_COMMAND

BaseTools/Tests: Use quotes around PYTHON_COMMAND

Commit ("2355f0c09c52 BaseTools: Fix check for ${PYTHON_COMMAND} in
Tests/GNUmakefile") fixed a latent issue in the BaseTools/Tests
Makefile, but inadvertently broke the BaseTools build for cases where
PYTHON_COMMAND is not set. As it turns out, running 'command' without a
command argument makes the invocation succeed, causing the empty
variable to be evaluated and called later.

Let's put double quotes around PYTHON_COMMAND in the invocation of
'command' and force it to fail when PYTHON_COMMAND is not set.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>