parser/metadecoders: Add empty /data JSON file as empty map

When fetching JSON files from the /data folder that are empty they will
be added as empty map[string]any. This makes sure that no empty JSON
file causes the site to crash because of a failed unmarshal. This
happens because empty is not a valid JSON string. It is therefore
important to check the lenght of the data before passing it to the JSON
unmarshal function.

Fixes #8601

parser/metadecoders/decoder.go

@@ -112,7 +112,7 @@ func (d Decoder) UnmarshalStringTo(data string, typ any) (any, error) {
 // Unmarshal will unmarshall data in format f into an interface{}.
 // This is what's needed for Hugo's /data handling.
 func (d Decoder) Unmarshal(data []byte, f Format) (any, error) {
-	if data == nil {
+	if data == nil || len(data) == 0 {
 		switch f {
 		case CSV:
 			return make([][]string, 0), nil

parser/metadecoders/decoder_test.go

@@ -122,6 +122,7 @@ func TestUnmarshalToInterface(t *testing.T) {
 	}{
 		{`[ "Brecker", "Blake", "Redman" ]`, JSON, []any{"Brecker", "Blake", "Redman"}},
 		{`{ "a": "b" }`, JSON, expect},
+		{``, JSON, map[string]any{}},
 		{`#+a: b`, ORG, expect},
 		{`#+DATE: <2020-06-26 Fri>`, ORG, map[string]any{"date": "2020-06-26"}},
 		{`a = "b"`, TOML, expect},