Disallow zero-length passphrases.

Fixes #2751
Closes #2762

res/values/strings.xml

@@ -251,6 +251,7 @@
     <!-- PassphraseChangeActivity -->
     <string name="PassphraseChangeActivity_passphrases_dont_match_exclamation">Passphrases don\'t match!</string>
     <string name="PassphraseChangeActivity_incorrect_old_passphrase_exclamation">Incorrect old passphrase!</string>
+    <string name="PassphraseChangeActivity_enter_new_passphrase_exclamation">Enter new passphrase!</string>
 
     <!-- DeviceProvisioningActivity -->
     <string name="DeviceProvisioningActivity_link_this_device">Link this device?</string>

src/org/thoughtcrime/securesms/PassphraseChangeActivity.java

@@ -96,6 +96,10 @@ public class PassphraseChangeActivity extends PassphraseActivity {
                        Toast.LENGTH_SHORT).show();
         this.newPassphrase.setText("");
         this.repeatPassphrase.setText("");
+      } else if (passphrase.equals("")) {
+        Toast.makeText(getApplicationContext(),
+                       R.string.PassphraseChangeActivity_need_new_passphrase_exclamation,
+                       Toast.LENGTH_SHORT).show();
       } else {
         MasterSecret masterSecret = MasterSecretUtil.changeMasterSecretPassphrase(this, original, passphrase);
         TextSecurePreferences.setPasswordDisabled(this, false);