Make DefaultValueSerializer null-safe for blobs
Make DefaultValueSerializer null-safe for blobs

This fixes an issue where a nullable blob column, e.g. `BlobColumn get logic => blob().nullable()();` will throw an error that looks like `NoSuchMethodError: The method 'cast' was called on null.`

(cherry picked from commit 87937ba89b28e956963555d7a456b61b67500043)

simolus3/moor,simolus3/drift,simolus3/drift,simolus3/moor,simolus3/drift,simolus3/drift,simolus3/moor,simolus3/moor,simolus3/drift,simolus3/moor,simolus3/drift,simolus3/moor,simolus3/drift,simolus3/moor