Adds logic to check for outputPath

Without this logic, NPE are possible

fixes #95

Author: tinder-maxwellelliott

src/main/java/com/bazel_diff/main.java

@@ -111,6 +111,10 @@ public Integer call() throws IOException {
             System.out.println("finalHashesJSONPath does not exist! Exiting");
             return ExitCode.USAGE;
         }
+        if (outputPath == null) {
+            System.out.println("outputPath was not provided! Exiting");
+            return ExitCode.USAGE;
+        }
         GitClient gitClient = new GitClientImpl(workspacePath);
         BazelClient bazelClient = new BazelClientImpl(
                 workspacePath,