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

meson.build

@@ -233,8 +233,10 @@ elif host_system == 'darwin'
   sysroot_args = [files('src/tools/darwin_sysroot'), get_option('darwin_sysroot')]
   pg_sysroot = run_command(sysroot_args, check:true).stdout().strip()
   message('darwin sysroot: @0@'.format(pg_sysroot))
-  cflags += ['-isysroot', pg_sysroot]
-  ldflags += ['-isysroot', pg_sysroot]
+  if pg_sysroot != ''
+    cflags += ['-isysroot', pg_sysroot]
+    ldflags += ['-isysroot', pg_sysroot]
+  endif
   # meson defaults to -Wl,-undefined,dynamic_lookup for modules, which we
   # don't want because a) it's different from what we do for autoconf, b) it
   # causes warnings starting in macOS Ventura