Fix split-string(nil) in gerrit-dashboard--cell-formatter

[whame]

If gerrit-dashboard is called and a reviewer-account-id cannot be found from (gerrit-get-accounts-alist),
gerrit-dashboard--cell-formatter calls split-string with a nil value. The following captured stack trace can be captured in this case (edited for readability):

  Debugger entered--Lisp error: (wrong-type-argument stringp nil)
    split-string(nil)
    ...
    gerrit-dashboard--cell-formatter(... "Reviewers")
    ...
    gerrit-dashboard--change-metadata-2-entry(...)
    ...
    gerrit-dashboard--get-data("has:draft")
    ...
    gerrit-dashboard--get-list-entries()
    gerrit-dashboard--refresh()
    gerrit-dashboard-mode()
    gerrit-dashboard()
    funcall-interactively(gerrit-dashboard)
    command-execute(gerrit-dashboard record)
    execute-extended-command(nil "gerrit-dashboard" "gerr")
    funcall-interactively(execute-extended-command nil "gerrit-dashboard" "gerr")
    command-execute(execute-extended-command)

The above was triggered for the "Reviewers" column in gerrit-dashboard--cell-formatter. This could for instance happen if a user is removed from the Gerrit instance's database but the change-metadata still list them (at least that's what I think has happened on my instance).

Fix this by instead returning "N/A" as name if reviewer-account-id can't be found in (gerrit-get-accounts-alist). There is only one other place where (gerrit-get-accounts-alist) is called and checked in a similar way, the "CC" column. While this column hasn't triggered this error, it doesn't hurt to be defensive and fix it here also.