Resolve "Relevance Ranking in Metadata"

Required MRs to be merged before

• codeabilitysharingpluginapi!2 (merged)
• !217 (merged)
• codeabilitysharingpluginapi!3 (merged)

Related to #439 (closed)

src/main/java/at/ac/uibk/gitsearch/service/GitlabService.java

@@ -148,15 +148,20 @@ public class GitlabService {
 
         RepositoryFile file;
         try (GitLabApi gitLabApi = gitLabRepository.getGitLabApi(tokenProvider.getGitLabAccessInfo());) {
-            file =
-                gitLabApi
-                    .getRepositoryFileApi()
-                    .getFile(
-                        Long.valueOf(exercise.getProject().getProject_id()),
-                        exerciseId.extendPath(filePath),
-                        exercise.getFile().getCommit_id()
-                    );
-            return new ByteArrayInputStream(file.getDecodedContentAsBytes());
+            if (exercise.getFile().getCommit_id() != null) {
+                file =
+                    gitLabApi
+                        .getRepositoryFileApi()
+                        .getFile(
+                            Long.valueOf(exercise.getProject().getProject_id()),
+                            exerciseId.extendPath(filePath),
+                            exercise.getFile().getCommit_id()
+                        );
+                return new ByteArrayInputStream(file.getDecodedContentAsBytes());
+            } else {
+                log.warn("commit_id must not be null for {}", exerciseId);
+                return null;
+            }
         } catch (GitLabApiException e) {
             log.warn(
                 "GitlabAPI Exception when looking for {} {} {} with message {}",