The `when' block would only return a string when the condition is
satisfied. If not, it returns `nil'. This is not acceptable, as
`postfix' is later used in a `concat'. This commit makes `postfix' an
empty string if the `when' condition fails in order to prevent this
concatenation issue.
This new function allows for a cleaner hook addition. It also moves
the check for the presence of the auto-id option to this function.
This allows the user to call org-unique-id on any org buffer,
regardless of the presence of auto-id and its value.
