Check for null rule name

Make sure that the rule name is not null here
fixes #78

Author: tinder-maxwellelliott

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

@@ -89,7 +89,7 @@ private byte[] createDigestForRule(
             digest.update(ruleInput.getBytes());
             BazelRule inputRule = allRulesMap.get(ruleInput);
             byte[] sourceFileDigest = getDigestForSourceTargetName(ruleInput, bazelSourcefileTargets);
-            if (inputRule != null && !inputRule.getName().equals(rule.getName())) {
+            if (inputRule != null && inputRule.getName() != null && !inputRule.getName().equals(rule.getName())) {
                 byte[] ruleInputHash = createDigestForRule(
                         inputRule,
                         allRulesMap,