build: add check for empty openssl-fips flag

Currently, when specifying the --openssl-fips flag without any path
, or an empty path, does not generate an error. If a path is specified
then the following error is generated:

ERROR: FIPS is not supported in this version of Node.js

This commit adds a check so that the error is generated even if the
path is empty.

PR-URL: https://github.com/nodejs/node/pull/25391


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>