Prevent crashes in `rebar3 as` with no tasks

checks on hd(...) and so on could not handle empty lists

src/rebar_prv_as.erl

@@ -33,9 +33,11 @@ init(State) ->
 -spec do(rebar_state:t()) -> {ok, rebar_state:t()} | {error, string()}.
 do(State) ->
     {Profiles, Tasks} = args_to_profiles_and_tasks(rebar_state:command_args(State)),
-    case Profiles of
-        [] ->
+    case {Profiles, Tasks} of
+        {[], _} ->
             {error, "At least one profile must be specified when using `as`"};
+        {_, []} ->
+            {error, "At least one task must be specified when using `as`"};
         _  ->
             warn_on_empty_profile(Profiles, State),
             State1 = rebar_state:apply_profiles(State, [list_to_atom(X) || X <- Profiles]),

test/rebar_as_SUITE.erl

@@ -14,6 +14,7 @@
          as_dir_name/1,
          as_with_task_args/1,
          warn_on_empty_profile/1,
+         error_on_empty_tasks/1,
          clean_as_profile/1]).
 
 -include_lib("common_test/include/ct.hrl").
@@ -33,7 +34,7 @@ all() -> [as_basic, as_multiple_profiles, as_multiple_tasks,
           as_multiple_profiles_multiple_tasks,
           as_comma_placement, as_comma_then_space,
           as_dir_name, as_with_task_args,
-          warn_on_empty_profile, clean_as_profile].
+          warn_on_empty_profile, error_on_empty_tasks, clean_as_profile].
 
 as_basic(Config) ->
     AppDir = ?config(apps, Config),
@@ -159,6 +160,20 @@ warn_on_empty_profile(Config) ->
     meck:unload(rebar_log),
     ok.
 
+error_on_empty_tasks(Config) ->
+    AppDir = ?config(apps, Config),
+
+    Name = rebar_test_utils:create_random_name("as_error_empty_"),
+    Vsn = rebar_test_utils:create_random_vsn(),
+    rebar_test_utils:create_app(AppDir, Name, Vsn, [kernel, stdlib]),
+
+    meck:new(rebar_log, [passthrough]),
+    rebar_test_utils:run_and_check(Config,
+                                   [],
+                                   ["as", "default"],
+                                   {error, "At least one task must be specified when using `as`"}),
+    ok.
+
 warn_match(App, History) ->
     lists:any(
         fun({_, {rebar_log,log, [warn, "No entry for profile ~s in config.",