Fixes #115787

Before this change sometimes when starting a selection gesture such as a long press drag or a double tap drag directly on the empty padding of a Scrollable the selection may not be set initially because there was nothing to select in the empty padding. This caused a crash when receiving subsequent edge update events because we believe the selection starts in the scrollable but we do not have any selection edges set.

To fix this I made _selectionStartsInScrollable nullable, this way a subsequent selection edge will not try to set _selectionStartsInScrollable again if it has already been set removing the need for the assert. handleSelectionEdgeUpdate previously assumed that if the cached edges were not set then we should set _selectionStartsInScrollable, the assert was used to verify that _selectionStartsInScrollable was false i.e. had not been set/changed from its initial value. By making _selectionStartsInScrollable nullable we can ensure a subsequent edge update event does not re-set the value if it has already been set previously.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.