fix for potential null pointer

Author: tinder-maxwellelliott

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

@@ -53,7 +53,7 @@ public Set<String> queryForImpactedTargets(Set<String> impactedTargets, String a
                                             .map(target -> String.format("'%s'", target))
                                             .collect(Collectors.joining(" + "));
         String query = "";
-        if (hashAllTargets) {
+        if (hashAllTargets != null && hashAllTargets) {
             query = targetQuery;
         } else {
             query = String.format("rdeps(//... except '//external:all-targets', %s)", targetQuery);