Ensure encrypting device name does not die on empty device name (#3066)

Ran into this on the Contributors guidelines setting up the development env.
registerSingleDevice does not pass a deviceName, so it fail every time.

libtextsecure/account_manager.js

@@ -47,6 +47,9 @@
       return this.server.requestVerificationSMS(number);
     },
     async encryptDeviceName(name, providedIdentityKey) {
+      if (!name) {
+        return null;
+      }
       const identityKey =
         providedIdentityKey ||
         (await textsecure.storage.protocol.getIdentityKeyPair());