Fix empty cabinet creation in CabFlush

When no files are added to a cabinet (total_files == 0), CabFlush
previously returned success without creating any file. This caused
WiX's file transfer system to fail with FileNotFoundException when
trying to move the expected cabinet to the output directory.

Now creates a valid empty cabinet file (cabinet with an empty folder)
and reports it back to the caller, matching the behavior expected by
WiX for declared-but-empty cabinets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: caesay

msi-interop/msi_cab.c

@@ -348,7 +348,25 @@ CabFlush(CABHANDLE       handle,
 
     guint total_files = ctx->files->len;
     if (total_files == 0) {
-        /* No files added -- nothing to write */
+        /* Create an empty cabinet file (valid cabinet with no files). */
+        GError *error = NULL;
+        gboolean ok = write_single_cabinet(ctx, 0, 0, ctx->output_path, &error);
+        g_clear_error(&error);
+
+        if (!ok)
+            return CAB_ERROR_FUNCTION_FAILED;
+
+        /* Report the single empty cabinet */
+        WCHAR *cab_name_w = utf8_to_utf16(ctx->cab_basename, NULL);
+        WCHAR *empty_token_w = utf8_to_utf16("", NULL);
+        g_ptr_array_add(ctx->result_cab_names, cab_name_w);
+        g_ptr_array_add(ctx->result_tokens, empty_token_w);
+
+        if (out_cabs != NULL && max_cabs > 0) {
+            out_cabs[0].cabinet_name     = (const wchar_t *)g_ptr_array_index(ctx->result_cab_names, 0);
+            out_cabs[0].first_file_token = (const wchar_t *)g_ptr_array_index(ctx->result_tokens, 0);
+        }
+        *out_count = 1;
         return CAB_SUCCESS;
     }