Remove redundant constant nil argument to gethash

Remove redundant constant nil argument to gethash

* lisp/cedet/ede/files.el (ede--directory-project-from-hash):
* lisp/emacs-lisp/edebug.el (edebug-unwrap*):
* lisp/emacs-lisp/testcover.el (testcover--copy-object1):
* lisp/net/zeroconf.el (zeroconf-get-service, zeroconf-resolve-service)
(zeroconf-register-service-browser, zeroconf-service-browser-handler)
(zeroconf-register-service-resolver):
* lisp/url/url-history.el (url-have-visited-url): Remove redundant
constant nil argument to gethash.

(cherry picked from commit 14cf4d538350fd2b1eda35101f5bb585f55e4659)