Fix markdownlint when there are no md files detected

Add an extra clause to the lint-doc.sh script and check if the list of
markdown files is empty when a merge results pipeline is run. If it is,
skip the markdownlint check altogether.

scripts/lint-doc.sh

@@ -66,8 +66,11 @@ then
 else
   MERGE_BASE=$(git merge-base ${CI_MERGE_REQUEST_TARGET_BRANCH_SHA} ${CI_MERGE_REQUEST_SOURCE_BRANCH_SHA})
   MD_DOC_PATH=$(git diff --name-only "${MERGE_BASE}..${CI_MERGE_REQUEST_SOURCE_BRANCH_SHA}" 'doc/*.md')
-  echo -e "Merged results pipeline detected. Testing only the following files:\n${MD_DOC_PATH}"
- fi
+  if [ -n "${MD_DOC_PATH}" ]
+  then
+    echo -e "Merged results pipeline detected. Testing only the following files:\n${MD_DOC_PATH}"
+  fi
+fi
 
 function run_locally_or_in_docker() {
   local cmd=$1
@@ -98,7 +101,12 @@ function run_locally_or_in_docker() {
 
 echo '=> Linting markdown style...'
 echo
-run_locally_or_in_docker 'markdownlint' "--config .markdownlint.json ${MD_DOC_PATH}"
+if [ -z "${MD_DOC_PATH}" ]
+then
+  echo "Merged results pipeline detected, but no markdown files found. Skipping."
+else
+  run_locally_or_in_docker 'markdownlint' "--config .markdownlint.json ${MD_DOC_PATH}"
+fi
 
 echo '=> Linting prose...'
 run_locally_or_in_docker 'vale' "--minAlertLevel error --output=JSON ${MD_DOC_PATH}" "ruby scripts/vale.rb"