narrow: Handle narrow links with empty topic names.

This commit ensures that narrow links involving
empty topic names, such as:

* #narrow/channel/1-general/topic//near/12345
* #narrow/channel/1-general/topic/

work correctly in the web client.

This change ensures proper navigation and behavior
for such links.

web/src/hash_util.ts

@@ -173,7 +173,7 @@ export function parse_narrow(hash: string[]): NarrowTerm[] | undefined {
 
         const raw_operand = hash[i + 1];
 
-        if (!raw_operand) {
+        if (raw_operand === undefined) {
             return undefined;
         }
 
@@ -183,6 +183,12 @@ export function parse_narrow(hash: string[]): NarrowTerm[] | undefined {
             operator = operator.slice(1);
         }
 
+        // We allow the empty string as a topic name.
+        // Any other operand being empty string is invalid.
+        if (operator !== "topic" && raw_operand === "") {
+            return undefined;
+        }
+
         const operand = decode_operand(operator, raw_operand);
         terms.push({negated, operator, operand});
     }

web/tests/hash_util.test.cjs

@@ -190,6 +190,18 @@ run_test("test_parse_narrow", () => {
     assert.deepEqual(hash_util.parse_narrow(["narrow", "stream", "42-bogus"]), [
         {negated: false, operator: "stream", operand: ""},
     ]);
+
+    // Empty string as a topic name is valid.
+    assert.deepEqual(hash_util.parse_narrow(["narrow", "stream", "99-frontend", "topic", ""]), [
+        {negated: false, operator: "stream", operand: frontend_id.toString()},
+        {negated: false, operator: "topic", operand: ""},
+    ]);
+
+    // Empty string used as an operand in other cases is invalid.
+    assert.deepEqual(
+        hash_util.parse_narrow(["narrow", "stream", "99-frontend", "topic", "", "near", ""]),
+        undefined,
+    );
 });
 
 run_test("test_channels_settings_edit_url", () => {