MnemonicCode: Add null check for passphrase.

The null check is added due to a likely undesirable outcome when supplying a null passphrase.
If allowed, the salt will be "mnemonicnull", when one would expect only "mnemonic" due to the
following from BIP39 "If a passphrase is not present, an empty string "" is used instead."

core/src/main/java/org/bitcoinj/crypto/MnemonicCode.java

@@ -34,6 +34,7 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 
+import static com.google.common.base.Preconditions.checkNotNull;
 import static org.bitcoinj.core.Utils.HEX;
 
 /**
@@ -119,6 +120,7 @@ public class MnemonicCode {
      * Convert mnemonic word list to seed.
      */
     public static byte[] toSeed(List<String> words, String passphrase) {
+        checkNotNull(passphrase, "A null passphrase is not allowed.");
 
         // To create binary seed from mnemonic, we use PBKDF2 function
         // with mnemonic sentence (in UTF-8) used as a password and

core/src/test/java/org/bitcoinj/crypto/MnemonicCodeTest.java

@@ -212,6 +212,12 @@ public class MnemonicCodeTest {
         mc.toMnemonic(entropy);
     }
 
+    @Test(expected = NullPointerException.class)
+    public void testNullPassphrase() throws Exception {
+        List<String> code = split("legal winner thank year wave sausage worth useful legal winner thank yellow");
+        MnemonicCode.toSeed(code, null);
+    }
+
     public static List<String> split(String words) {
         return new ArrayList<String>(Arrays.asList(words.split("\\s+")));
     }