meson: Fix support for empty darwin sysroot
The -isysroot options should only be added if the sysroot resolved to
a nonempty string. This matches the behavior in src/template/darwin
(also documented in installation.sgml).
Discussion: https://www.postgresql.org/message-id/flat/
60765bf0-5027-4b23-9f78-
4a365d28823f%40enterprisedb.com