Return a non-null String value for a key. This method is included
for backwards compatibility with the removed class AbstractMapLabel.
It is guaranteed to not return null; if the key is not present or
has a null value, it returns the empty string (""). It is only valid to
call this method when key is paired with a value of type String.
KEY - A key type with a String value
key - The key to return the value of.
"" if the key is not in the map or has the value null
and the String value of the key otherwise