configure: simplify calls to meson_quote

meson_quote assumes a non-empty argument list, and incorrectly returns a
one-entry array if passed nothing.  Move the check for an empty argument
list from the invocations to the function itself.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>