Fix up empty namespaces JSON unmarshalling error

At present, if the namespaces endpoint returns 0 bytes, there is still
an attempt to unmarshal it, resulting in an unexected end of JSON input
error being passed down.

Signed-off-by: Paul Mundt <paul.mundt@adaptant.io>

types/function_list_builder.go

@@ -54,6 +54,10 @@ func (s *FunctionLookupBuilder) getNamespaces() ([]string, error) {
 			return namespaces, err
 		}
 
+		if len(bytesOut) == 0 {
+			return namespaces, nil
+		}
+
 		err = json.Unmarshal(bytesOut, &namespaces)
 		if err != nil {
 			return namespaces, err