fix(js): Fix incorrect beforeSendSpan callback type (#13906)

Implicitly reported via
getsentry/sentry-javascript#16439 that according
to the type in the options page, the `beforeSendSpan` callback can
return `null`. This is no longer the case since v9 of the SDK.

Author: Lms24

docs/platforms/javascript/common/configuration/options.mdx

@@ -385,10 +385,10 @@ This function is called with a transaction event object, and can return a modifi
 
 </SdkOption>
 
-<SdkOption name="beforeSendSpan" type='(span: SpanJSON) => SpanJSON | null'>
+<SdkOption name="beforeSendSpan" type='(span: SpanJSON) => SpanJSON'>
 
 This function is called with a serialized span object, and can return a modified span object. This might be useful for manually stripping PII from spans.
-This function is only called for root spans and all children.
+This function is called for root spans as well as for all child spans.
 If you want to drop the root span, including all of its child spans, use [`beforeSendTransaction`](#beforeSendTransaction) instead.
 
 Please note that the `span` you receive as an argument is a serialized object, not a `Span` class instance.