meson: error out if platforms contains empty string

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110939
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
10 jobs for !1139 with meson-fix-empty-platforms in 10 minutes and 46 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #377126
debian

00:00:22

 
  Build+Test
passed #377128
meson-clang

00:10:23

passed #377132
meson-clover

00:05:31

passed #377131
meson-main

00:04:26

passed #377127
meson-swr-glvnd

00:05:28

passed #377133
meson-vulkan

00:01:48

passed #377135
scons-llvm

00:03:43

passed #377134
scons-nollvm

00:03:34

passed #377129
scons-swr

00:05:23

passed #377130
scons-win64

00:02:14