Cast `$length` to an actual `int` in TLengthDatabaseTableColumn::length()

Cast `$length` to an actual `int` in TLengthDatabaseTableColumn::length()

When the object is being initialized from the existing structure in the
database, the length will be passed as a string and was previously stored as-is
within the object.

This violates the existing PHPDoc type declaration and breaks consumers that
use a strict comparison (`===`) to check the length, notably
`YearDatabaseTableColumn`.

Fix this by casting the passed parameter to an actual `int`. This should be
adjusted to a proper parameter type in a future version.

Resolves #4594

[Tim: Adjusted commit message]