(occur-read-primary-args): Set default to the car of

(occur-read-primary-args): Set default to the car of
regexp-history and display it in the prompt, but don't add to the
list of minibuffer defaults.  Bind history-add-new-input to nil to
not add automatically `default'.  For empty input return `default'.
Otherwise, add `input' to regexp-history and return it.
(occur-1): Signal an error for the empty regexp.