Correctly handle empty hash as default named argument

```
def func(arg: {})
  p arg
end
```

This used to work in earlier mruby versions, but broke somewhere recently.

mrbgems/mruby-compiler/core/parse.y

@@ -4563,13 +4563,13 @@ f_label         : tIDENTIFIER tLABEL_TAG
                     {
                       $$ = $1;
                       local_nest(p);
-                      p->lstate = EXPR_ARG;  /* make newlines significant after label */
+                      p->lstate = EXPR_MID;  /* make newlines significant after label */
                     }
                 | tNUMPARAM tLABEL_TAG
                     {
                       $$ = intern_numparam($1);
                       local_nest(p);
-                      p->lstate = EXPR_ARG;  /* make newlines significant after label */
+                      p->lstate = EXPR_MID;  /* make newlines significant after label */
                     }
                 ;
 

test/t/syntax.rb

@@ -680,6 +680,20 @@ assert 'keyword arguments' do
   assert_equal [{a: 1}, {b: 2}], m({a: 1}, b: 2)
   assert_raise(ArgumentError) { m({a: 1}, {b: 2}) }
 
+  def m1(a: {}) a end
+  assert_equal({}, m1)
+  assert_equal(:abc, m1(a: :abc))
+
+  def m2(a: +1) a end
+  assert_equal(1, m2)
+
+  assert_nothing_raised do
+    def m3 arg:
+      123
+    end
+  end
+  assert_equal(123, m3(arg: 456))
+
   def m(*, a:) a end
   assert_equal 1, m(a: 1)
   assert_equal 3, m(1, 2, a: 3)