* isearch.el (isearch--describe-regexp-mode): The `description' var
needs to always be a string. Add the missing default case for the
cond form that ensures that.
Before this bug fix, for the events when `regexp-function' and
`search-default-mode' both were nil, `description' also stayed nil. So
when `space-before' was non-nil, the "non-string" `description'
(with a value of nil) got passed as an argument to
`replace-regexp-in-string' (where a string was expected). That caused
the error described in Bug#23038.