honor removing a time constraint

Looks like a bug got introduced with the great UI rewrite: when you add
a (start or end) time constraint, then remove one, the change wouldn't
be reflected. Within CalendarTSRange, it used null to mean to keep a
value, and || to check if it was null. These meant empty strings turned
into the existing value, instead of no constraint as they should be.
This was unnecessarily clever; stop doing that.

Also keep the console logging in the deployed config; it's harmless and
eases debugging.

webpack/prod.config.js

@@ -76,7 +76,6 @@ module.exports = (env, args) => {
                   )),
               uglifyOptions: {
                 compress: {
-                  drop_console: true, // Remove all console.log etc.
                   keep_infinity: true, // Do not change to 1/0
                   warnings: false, // Do not warn when dropping
                 },