[Integrated] RFR: 339: Only show link to CONTRIBUTING.md if it exists

[Erik Helin]

Changeset: 870bf786
Author:    Erik Helin <ehelin at openjdk.org>
Date:      2020-03-30 14:28:00 +0000
URL:       https://git.openjdk.java.net/skara/commit/870bf786

339: Only show link to CONTRIBUTING.md if it exists

Reviewed-by: rwestberg

! bots/pr/src/main/java/org/openjdk/skara/bots/pr/CheckRun.java
! bots/pr/src/test/java/org/openjdk/skara/bots/pr/IntegrateTests.java