Make sure the parameter is not truncated if the value is empty

src/icalendar/parser.py

@@ -338,7 +338,7 @@ class Contentline(str):
                 raise ValueError('Key name is required')
             validate_token(name)
             if not value_split:
-                value_split = i
+                value_split = i + 1
             if not name_split or name_split + 1 == value_split:
                 raise ValueError('Invalid content line')
             params = Parameters.from_ical(st[name_split + 1: value_split],