Since the 3.0.10 fix for 'Invalid Prefix Names Allowed' (
PHPBB3-9066), they have become a required field for installation.
If you clear the field (do not want a prefix), it will not progress to the next step with the error: "The prefix you entered is invalid".
This is caused by the table_prefix regex matching a string that starts with a-zA-Z and then 0 or more other valid characters and/or there being no logic to skip this check if the field is empty.
The correct behaviour would be skip the regex test against this field if it is empty.