[client-sync] Fix contact ranking error

Summary:
This was showing up in Sentry. Check to see if the recipient has an email
before calling toLowerCase.

Test Plan: yolo

Reviewers: halla, juan

Reviewed By: juan

Differential Revision: https://phab.nylas.com/D4423

packages/client-sync/src/local-api/routes/contacts.js

@@ -16,6 +16,9 @@ const calculateContactScores = (messages, result) => {
   for (const message of messages) {
     const weight = getMessageWeight(message, now);
     for (const recipient of message.getRecipients()) {
+      if (!recipient.email) {
+        continue;
+      }
       const email = recipient.email.toLowerCase();
       result[email] = result[email] ? (result[email] + weight) : weight;
     }