meson: Do not try to build module for empty per-target hw/ directory

In commit 83d5db95d3 ("meson: Allow system binaries to not
have target-specific units") we allowed targets with empty
target_system_arch[] source set, but missed hw_arch[] could
also be empty when building modules.

Skip such case, otherwise due to commit a1ced48754
("hw/microblaze: Build files once"), building with
--enable-modules triggers:

  ../meson.build:4034:14: ERROR: Key microblaze is not in the dictionary.

Fixes: a1ced48754 ("hw/microblaze: Build files once")
Reported-by: Frederic Bezies <fredbezies@gmail.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3272
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20260116131817.38009-1-philmd@linaro.org>

meson.build

@@ -3957,6 +3957,11 @@ if enable_modules
   foreach target : target_dirs
     if target.endswith('-softmmu')
       config_target = config_target_mak[target]
+      arch = config_target['TARGET_NAME'] == 'sparc64' ? 'sparc64' : config_target['TARGET_BASE_ARCH']
+      if not hw_arch.has_key(arch)
+        continue
+      endif
+
       config_devices_mak = target + '-config-devices.mak'
       modinfo_src = custom_target('modinfo-' + target + '.c',
                                   output: 'modinfo-' + target + '.c',
@@ -3966,8 +3971,6 @@ if enable_modules
 
       modinfo_lib = static_library('modinfo-' + target + '.c', modinfo_src)
       modinfo_dep = declare_dependency(link_with: modinfo_lib)
-
-      arch = config_target['TARGET_NAME'] == 'sparc64' ? 'sparc64' : config_target['TARGET_BASE_ARCH']
       hw_arch[arch].add(modinfo_dep)
     endif
   endforeach