meson: disable rc-local-generator when an empty string is specified to rc-local= meson option

In such case, the generator is meaningless. Let's kill it.

man/rules/meson.build

@@ -1071,7 +1071,10 @@ manpages = [
   '8',
   ['systemd-random-seed'],
   'ENABLE_RANDOMSEED'],
- ['systemd-rc-local-generator', '8', ['rc-local.service'], 'HAVE_SYSV_COMPAT'],
+ ['systemd-rc-local-generator',
+  '8',
+  ['rc-local.service'],
+  'HAVE_SYSV_COMPAT HAVE_SYSV_RC_LOCAL'],
  ['systemd-remount-fs.service', '8', ['systemd-remount-fs'], ''],
  ['systemd-repart', '8', ['systemd-repart.service'], 'ENABLE_REPART'],
  ['systemd-resolved.service', '8', ['systemd-resolved'], 'ENABLE_RESOLVE'],

man/systemd-rc-local-generator.xml

@@ -6,7 +6,7 @@
 ]>
 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
 
-<refentry id="systemd-rc-local-generator" conditional='HAVE_SYSV_COMPAT'>
+<refentry id="systemd-rc-local-generator" conditional='HAVE_SYSV_COMPAT HAVE_SYSV_RC_LOCAL'>
   <refentryinfo>
     <title>systemd-rc-local-generator</title>
     <productname>systemd</productname>