changeURLMiddleware: respect pathnameShouldBe: ""

Prompted by @jameshadfield's review comment.¹
Fixes bug where ` pathnameShouldBe: ""` was ignored because `""` is
falsey. I chose to check `typeof` instead of `action.hasOwnProperty`
because I found cases where `pathnameShouldBe` can potentially
be set to `undefined`.²

¹ <#1804 (comment)>
² <https://github.com/nextstrain/auspice/blob/84a15484acabee10a65d2d4138f83e51ae068b51/src/actions/loadData.js#L128>

src/middleware/changeURL.js

@@ -229,7 +229,7 @@ export const changeURLMiddleware = (store) => (next) => (action) => {
   /* second switch: path change */
   switch (action.type) {
     case types.CLEAN_START:
-      if (action.pathnameShouldBe && !action.narrative) {
+      if (typeof action.pathnameShouldBe === "string" && !action.narrative) {
         pathname = action.pathnameShouldBe;
         break;
       }