[PATCH 1/2] ci: work around old records of GitHub runs

["Johannes Schindelin via GitGitGadget" <gitgitgadget@xxxxxxxxx>]

From: Johannes Schindelin <johannes.schindelin@xxxxxx>

Apparently older GitHub runs at least _sometimes_ lack information about
the `head_commit` (and therefore the `ci-config` check will fail with
"TypeError: Cannot read property 'tree_id' of null") in the check added
in 7d78d5fc1a9 (ci: skip GitHub workflow runs for already-tested
commits/trees, 2020-10-08).

Let's work around this by adding a defensive condition.

Reported-by: Philippe Blain <levraiphilippeblain@xxxxxxxxx>
Signed-off-by: Johannes Schindelin <johannes.schindelin@xxxxxx>
---
 .github/workflows/main.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 0a9acb6a19..d4298878f5 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -65,7 +65,7 @@ jobs:
                 core.setOutput('enabled', ' but skip');
                 break;
               }
-              if (tree_id === run.head_commit.tree_id) {
+              if (run.head_commit && tree_id === run.head_commit.tree_id) {
                 core.warning(`Successful run for the tree ${tree_id}: ${run.html_url}`);
                 core.setOutput('enabled', ' but skip');
                 break;
-- 
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