add: warn when creating empty virtual packages

see alpine/aports#11736

[TT: minor stylistic change]

src/app_add.c

@@ -147,6 +147,8 @@ static int add_main(void *ctx, struct apk_ctx *ac, struct apk_string_array *args
 
 		virtdep.result_mask = APK_VERSION_EQUAL;
 		virtdep.version = virtpkg->version;
+
+		if (!args->num) apk_warn(out, "creating empty virtual package");
 	}
 
 	foreach_array_item(parg, args) {

test/basic8.test

@@ -2,5 +2,6 @@
 --no-network
 add -t .virtual
 @EXPECT
+WARNING: creating empty virtual package
 (1/1) Installing .virtual (20190603.131426)
 OK: 0 MiB in 0 packages