Checking invalid email gives an exception

Looks like we can end up in a state where an invalid author email is passed to jcheck from a PR.

[00:08][FINER] issue: author.email is null or empty
[00:08][FINER] THROWjava.lang.IllegalStateException: Invalid author email: name <>
at org.openjdk.skara.bots.pr/org.openjdk.skara.bots.pr.PullRequestCheckIssueVisitor.visit(PullRequestCheckIssueVisitor.java:161)
at org.openjdk.skara.jcheck/org.openjdk.skara.jcheck.AuthorEmailIssue.accept(AuthorEmailIssue.java:32)
at org.openjdk.skara.bots.pr/org.openjdk.skara.bots.pr.CheckablePullRequest.executeChecks(CheckablePullRequest.java:167)
at org.openjdk.skara.bots.pr/org.openjdk.skara.bots.pr.CheckRun.checkStatus(CheckRun.java:739)
at org.openjdk.skara.bots.pr/org.openjdk.skara.bots.pr.CheckRun.execute(CheckRun.java:88)
at org.openjdk.skara.bots.pr/org.openjdk.skara.bots.pr.CheckWorkItem.run(CheckWorkItem.java:186)
at org.openjdk.skara.bot/org.openjdk.skara.bot.BotRunner$RunnableWorkItem.run(BotRunner.java:79)