getNearOperandFromLink [nfc]: Return null for none instead of zero

NFC because we always drop the return value of this value on the
floor.

Re: whether 0 can actually be the ID of a real message…see
  https://chat.zulip.org/#narrow/stream/19-documentation/topic/IDs.20being.20zero/near/1112947
where Tim says:

> Yeah, @_**Anders Kaseorg** is correct. The server has an
> invariant that it won't generate an ID of 0 but it's possible to
> create such an entry via raw database manipulation.

Author: chrisbobbe

src/message/messagesActions.js

@@ -38,7 +38,7 @@ export const messageLinkPress =
     if (narrow) {
       // This call is OK: `narrow` is truthy, so isNarrowLink(…) was true.
       const nearOperand = getNearOperandFromLink(href, auth.realm);
-      dispatch(doNarrow(narrow, nearOperand));
+      dispatch(doNarrow(narrow, nearOperand ?? 0));
     } else if (!isUrlOnRealm(href, auth.realm)) {
       openLinkWithUserPreference(href, getGlobalSettings());
     } else {

src/utils/__tests__/internalLinks-test.js

@@ -420,8 +420,8 @@ describe('getNarrowFromLink', () => {
 
 describe('getNearOperandFromLink', () => {
   test('not message link', () => {
-    expect(getNearOperandFromLink('https://example.com/#narrow/is/private', realm)).toBe(0);
-    expect(getNearOperandFromLink('https://example.com/#narrow/stream/jest', realm)).toBe(0);
+    expect(getNearOperandFromLink('https://example.com/#narrow/is/private', realm)).toBe(null);
+    expect(getNearOperandFromLink('https://example.com/#narrow/stream/jest', realm)).toBe(null);
   });
 
   test('`near` is the only operator', () => {

src/utils/internalLinks.js

@@ -227,13 +227,13 @@ export const getNarrowFromLink = (
 
 /**
  * From a URL and realm with `isNarrowLink(url, realm) === true`, give
- *   message_id if the URL has /near/{message_id}, otherwise give zero.
+ *   message_id if the URL has /near/{message_id}, otherwise give null.
  *
  * This performs a call to `new URL` and therefore may take a fraction of a
  * millisecond.  Avoid using in a context where it might be called more than
  * 10 or 100 times per user action.
  */
-export const getNearOperandFromLink = (url: string, realm: URL): number => {
+export const getNearOperandFromLink = (url: string, realm: URL): number | null => {
   // isNarrowLink(…) is true, by jsdoc, so this call is OK.
   const hashSegments = getHashSegmentsFromNarrowLink(url, realm);
 
@@ -249,7 +249,7 @@ export const getNearOperandFromLink = (url: string, realm: URL): number => {
       && str === 'near',
   );
   if (nearOperatorIndex < 0) {
-    return 0;
+    return null;
   }
 
   // We expect an operand to directly follow its operator.
@@ -258,7 +258,7 @@ export const getNearOperandFromLink = (url: string, realm: URL): number => {
   const nearOperandStr = hashSegments[nearOperandIndex];
   // Must look like a message ID
   if (!/^[0-9]+$/.test(nearOperandStr)) {
-    return 0;
+    return null;
   }
 
   return parseInt(nearOperandStr, 10);