Date._<format>(nil) should return an empty Hash

Fix: #39

This is how versions previous to 3.2.1 behaved and Active Support
currently rely on this behavior.

https://github.com/rails/rails/blob/90357af08048ef5076730505f6e7b14a81f33d0c/activesupport/lib/active_support/values/time_zone.rb#L383-L384

Any Rails application upgrading to date `3.2.1` might run into unexpected errors.

ext/date/date_core.c

@@ -4342,6 +4342,8 @@ get_limit(VALUE opt)
 static void
 check_limit(VALUE str, VALUE opt)
 {
+    if (NIL_P(str)) return;
+
     StringValue(str);
     size_t slen = RSTRING_LEN(str);
     size_t limit = get_limit(opt);

test/date/test_date_parse.rb

@@ -848,6 +848,9 @@ def test__iso8601
 
     h = Date._iso8601('')
     assert_equal({}, h)
+
+    h = Date._iso8601(nil)
+    assert_equal({}, h)
   end
 
   def test__rfc3339
@@ -863,6 +866,9 @@ def test__rfc3339
 
     h = Date._rfc3339('')
     assert_equal({}, h)
+
+    h = Date._rfc3339(nil)
+    assert_equal({}, h)
   end
 
   def test__xmlschema
@@ -945,6 +951,9 @@ def test__xmlschema
 
     h = Date._xmlschema('')
     assert_equal({}, h)
+
+    h = Date._xmlschema(nil)
+    assert_equal({}, h)
   end
 
   def test__rfc2822
@@ -977,6 +986,9 @@ def test__rfc2822
 
     h = Date._rfc2822('')
     assert_equal({}, h)
+
+    h = Date._rfc2822(nil)
+    assert_equal({}, h)
   end
 
   def test__httpdate
@@ -997,6 +1009,9 @@ def test__httpdate
 
     h = Date._httpdate('')
     assert_equal({}, h)
+
+    h = Date._httpdate(nil)
+    assert_equal({}, h)
   end
 
   def test__jisx0301
@@ -1073,6 +1088,9 @@ def test__jisx0301
 
     h = Date._jisx0301('')
     assert_equal({}, h)
+
+    h = Date._jisx0301(nil)
+    assert_equal({}, h)
   end
 
   def test_iso8601