fix: Correct output path with failed builds

Previously, if a build failed to produce any output we would try to
write "None" to the output file. This doesn't work, causing the job to
error. Instead, we should skip writing the output and preserve the
original "failed" status.

buildbot_nix/__init__.py

@@ -339,6 +339,11 @@ class UpdateBuildOutput(steps.BuildStep):
         if not pr and props.getProperty("branch") != self.project.default_branch:
             return util.SKIPPED
 
+        out_path = props.getProperty("out_path")
+
+        if not out_path: # if, e.g., the build fails and doesn't produce an output
+            return util.SKIPPED
+
         owner = Path(props.getProperty("owner"))
         repo = Path(props.getProperty("repository_name"))
 
@@ -355,7 +360,6 @@ class UpdateBuildOutput(steps.BuildStep):
 
         file.parent.mkdir(parents=True, exist_ok=True)
 
-        out_path = props.getProperty("out_path")
         file.write_text(out_path)
 
         return util.SUCCESS