fix(discover): Prevent TypeError when decoding null fields from URL

[sentry]

This PR addresses a TypeError: Cannot read properties of null (reading 'match') that occurred when malformed URL query parameters for fields were present, specifically when an empty field parameter was included (e.g., &field=title&field).

Root Cause:
decodeFields in static/app/utils/discover/eventView.tsx uses decodeList to parse URL query parameters. When a query string like &field=title&field is encountered, decodeList can return an array containing null elements (e.g., ['title', null]). decodeFields then processed these null elements without validation, passing them downstream. Eventually, these null values reached functions like parseFunction in static/app/utils/discover/fields.tsx, which expected a string and attempted to call .match() on null, leading to the TypeError.

Solution:
An if (!field) return; guard has been added within the forEach loop in decodeFields. This ensures that any null or falsy field entries returned by decodeList are filtered out before being added to the parsed array. This prevents null values from propagating to downstream functions that expect string inputs, thereby resolving the TypeError.

Legal Boilerplate

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Fixes JAVASCRIPT-3AQT

_{Comment @sentry <feedback> on this PR to have Autofix iterate on the changes.}