handle empty Tuple{} in _methods_by_ftype (#46685)

This is silly, but some packages try very hard to precompile everything
including this, even though this is invalid.

Fix #46295

Author: vtjnash

src/gf.c

@@ -2004,7 +2004,7 @@ JL_DLLEXPORT jl_value_t *jl_matching_methods(jl_tupletype_t *types, jl_value_t *
     if (ambig != NULL)
         *ambig = 0;
     jl_value_t *unw = jl_unwrap_unionall((jl_value_t*)types);
-    if (jl_is_tuple_type(unw) && jl_tparam0(unw) == jl_bottom_type)
+    if (jl_is_tuple_type(unw) && (unw == (jl_value_t*)jl_emptytuple_type || jl_tparam0(unw) == jl_bottom_type))
         return (jl_value_t*)jl_an_empty_vec_any;
     if (mt == jl_nothing)
         mt = (jl_value_t*)jl_method_table_for(unw);

test/reflection.jl

@@ -1010,3 +1010,5 @@ ambig_effects_test(a, b) = 1
     @test (Base.infer_effects(setfield!, ()); true) # `builtin_effects` shouldn't throw on empty `argtypes`
     @test (Base.infer_effects(Core.Intrinsics.arraylen, ()); true) # `intrinsic_effects` shouldn't throw on empty `argtypes`
 end
+
+@test Base._methods_by_ftype(Tuple{}, -1, Base.get_world_counter()) == Any[]