Attempt to fix issue with -a flag

Fixes a null pointer dereference when using the hash all targets flag `-a`

Author: tinder-maxwellelliott

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

@@ -44,7 +44,9 @@ public String getName() {
     @Override
     public byte[] getDigest() throws NoSuchAlgorithmException {
         MessageDigest finalDigest = MessageDigest.getInstance("SHA-256");
-        finalDigest.update(digest);
+        if (digest != null) {
+            finalDigest.update(digest);
+        }
         finalDigest.update(name.getBytes());
         return finalDigest.digest();
     }