Support pages without front matter

* Page without front matter now treated same as a page with empty front matter.
* Test cases added to cover this and repro issue #4320.
* Type safety of front matter code improved.

Fixes #4320

commands/hugo.go

@@ -1231,9 +1231,7 @@ func (c *commandeer) isThemeVsHugoVersionMismatch() (mismatch bool, requiredMinV
 		return
 	}
 
-	config := tomlMeta.(map[string]interface{})
-
-	if minVersion, ok := config["min_version"]; ok {
+	if minVersion, ok := tomlMeta["min_version"]; ok {
 		return helpers.CompareVersion(minVersion) > 0, fmt.Sprint(minVersion)
 	}
 

commands/import_jekyll.go

@@ -255,7 +255,7 @@ func loadJekyllConfig(fs afero.Fs, jekyllRoot string) map[string]interface{} {
 		return nil
 	}
 
-	return c.(map[string]interface{})
+	return c
 }
 
 func createConfigFromJekyll(fs afero.Fs, inpath string, kind string, jekyllConfig map[string]interface{}) (err error) {

commands/undraft.go

@@ -99,7 +99,7 @@ func undraftContent(p parser.Page) (bytes.Buffer, error) {
 	var isDraft, gotDate bool
 	var date string
 L:
-	for k, v := range meta.(map[string]interface{}) {
+	for k, v := range meta {
 		switch k {
 		case "draft":
 			if !v.(bool) {

commands/undraft_test.go

@@ -69,7 +69,7 @@ func TestUndraftContent(t *testing.T) {
 				t.Errorf("[%d] unexpected error %q", i, err)
 				continue
 			}
-			for k, v := range meta.(map[string]interface{}) {
+			for k, v := range meta {
 				if k == "draft" {
 					if v.(bool) {
 						t.Errorf("[%d] Expected %q to be \"false\", got \"true\"", i, k)