Fail if no keypair is set

acme4j-client/src/main/java/org/shredzone/acme4j/Account.java

@@ -33,6 +33,10 @@ public class Account {
      *            {@link KeyPair} that identifies the account.
      */
     public Account(KeyPair keyPair) {
+        if (keyPair == null) {
+            throw new NullPointerException("keypair must be set");
+        }
+
         this.keyPair = keyPair;
     }