fix: do not include context fields with null value

[ivarconr]

Currently we are looping over the context fields and add all of them with their respective value to as URL parameters. searchParams.append(..) will gladly convert null and undefined and convert these into string values, which is kind of surprising.

const urlWithQuery = new URL("https://getunleash.io");

// Using null
urlWithQuery.searchParams.append('a', null);
urlWithQuery.searchParams.has('a');
> true
urlWithQuery.searchParams.get('a');
> "null"

// using undefined
urlWithQuery.searchParams.append('b', undefined);
urlWithQuery.searchParams.has('b');
> true
urlWithQuery.searchParams.get('b');
> "undefined"

this PR adds filtering of context fields which has value null or undefined.

[FredrikOseberg]

LGTM