msglist render [nfc]: Explicitly note an undefined Flow misses.

Kind of puzzling that Flow doesn't complain about this one on its
own!  As ESLint points out in the rule that we have to disable here,
the `= undefined` has no effect on runtime behavior -- it just
states explicitly what the code was already doing anyway.

But once we do make it explicit, Flow starts noticing that the
`undefined` was propagating into boolean expressions and to an
argument passed to `isSameRecipient` -- which also justifies a
check at the top of that function which by its types had looked
dead.  Fix that function's signature, and booleanize the value
when using as a boolean.

This Flow behavior is concerning enough that we should probably
just stop saying `let foo;`, and always explicitly initialize to
`undefined` when that's what we mean.  (The code is potentially
a bit clearer that way anyway.)  We'll do that next.

I've also reported the Flow issue upstream, as:
  facebook/flow#8526

src/message/renderMessages.js

@@ -10,11 +10,13 @@ export default (
 ): RenderedSectionDescriptor[] => {
   const showHeader = !isPmNarrow(narrow) && !isTopicNarrow(narrow);
 
-  let prevItem;
+  // eslint-disable-next-line no-undef-init
+  let prevItem = undefined;
   const sections = [{ key: 0, data: [], message: {} }];
   messages.forEach(item => {
     const diffDays =
-      prevItem && !isSameDay(new Date(prevItem.timestamp * 1000), new Date(item.timestamp * 1000));
+      !!prevItem
+      && !isSameDay(new Date(prevItem.timestamp * 1000), new Date(item.timestamp * 1000));
     if (!prevItem || diffDays) {
       sections[sections.length - 1].data.push({
         key: `time${item.timestamp}`,
@@ -23,6 +25,7 @@ export default (
         firstMessage: item,
       });
     }
+
     const diffRecipient = !isSameRecipient(prevItem, item);
     if (showHeader && diffRecipient) {
       sections.push({
@@ -37,7 +40,7 @@ export default (
     //   values that lack it; which is fine once the release that adds it
     //   has been out for a few weeks.
     const shouldGroupWithPrev =
-      !diffRecipient && !diffDays && prevItem && prevItem.sender_email === item.sender_email;
+      !diffRecipient && !diffDays && !!prevItem && prevItem.sender_email === item.sender_email;
 
     sections[sections.length - 1].data.push({
       key: item.id,

src/utils/recipient.js

@@ -339,8 +339,8 @@ export const pmTypingKeyFromRecipients = (
 ): string => pmTypingKeyFromPmKeyIds(filterRecipientsAsUserIds(recipients, ownUserId));
 
 export const isSameRecipient = (
-  message1: Message | Outbox,
-  message2: Message | Outbox,
+  message1: Message | Outbox | void,
+  message2: Message | Outbox | void,
 ): boolean => {
   if (message1 === undefined || message2 === undefined) {
     return false;