fix(discover): Prevent homepage crash on unmapped dataset param

[sentry]

Fixes a TypeError: Cannot read properties of undefined (reading 'environment') that occurred on /explore/discover/homepage/.

Root Cause:
When hasDatasetSelector is enabled and no saved homepage query exists, checkEventView in static/app/views/discover/results.tsx attempts to get a default event view using DEFAULT_EVENT_VIEW_MAP[value]. If the value (derived from the URL's ?dataset= parameter) does not correspond to one of the few keys in DEFAULT_EVENT_VIEW_MAP (e.g., DISCOVER, ERRORS, TRANSACTIONS), the lookup returns undefined. This undefined then propagates through getSavedQueryWithDataset(undefined) and is passed as the newQuery argument to EventView.fromNewQueryWithLocation, causing a crash when newQuery.environment is accessed.

Fix:
Added a nullish coalescing operator (?? DEFAULT_EVENT_VIEW) to the DEFAULT_EVENT_VIEW_MAP[value] lookup in static/app/views/discover/results.tsx. This ensures that if an unrecognized dataset value is encountered, it gracefully falls back to DEFAULT_EVENT_VIEW, guaranteeing a valid NewQuery object is always passed to EventView.fromNewQueryWithLocation.

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Fixes JAVASCRIPT-38Z6

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[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	95.43%	95.43%	±0%
Typed	138,616	138,616	±0
Untyped	6,640	6,640	±0

🔍 1 new type safety issue introduced

Type assertions (as) (1 new)

File	Line	Detail
static/app/views/discover/results.tsx	445	as NewQuery — getSavedQueryWithDataset( DEFAULT_EVENT_VIEW_MAP[value] ?? DEFAULT_EVENT_VIEW )…

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