tests: don't build audio tests when no audio drivers are enabled

When there are no audio drivers configure fails with "ERROR: Command
cannot have '@INPUT@', since no input files were specified".

Fixes: 3220b38a8d ("tests: start manual audio backend test")
Signed-off-by: Anton Kuchin <antonkuchin@nebius.com>
Message-ID: <DB8P190MB07142E1BA8DEEA8B2E41D5B2DB57A@DB8P190MB0714.EURP190.PROD.OUTLOOK.COM>
[ Marc-André - use empty modinfo stub ]
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

tests/audio/meson.build

@@ -6,12 +6,14 @@ endif
 
 modinfo_dep = not_found
 if enable_modules
-    modinfo_src = custom_target('modinfo.c',
-                                output: 'modinfo.c',
-                                input: audio_modinfo_files,
-                                command: [modinfo_generate, '--skip-missing-deps', '@INPUT@'],
-                                capture: true)
-
+    modinfo_src = 'modinfo-stub.c'
+    if audio_modinfo_files.length() != 0
+        modinfo_src = custom_target('modinfo.c',
+                                    output: 'modinfo.c',
+                                    input: audio_modinfo_files,
+                                    command: [modinfo_generate, '--skip-missing-deps', '@INPUT@'],
+                                    capture: true)
+    endif
     modinfo_lib = static_library('modinfo.c', modinfo_src)
     modinfo_dep = declare_dependency(link_with: modinfo_lib)
 endif

tests/audio/modinfo-stub.c

@@ -0,0 +1,5 @@
+#include "qemu/osdep.h"
+#include "qemu/module.h"
+const QemuModinfo qemu_modinfo[] = {
+  { /* end of list */ }
+};