Handle NULL for max_filesize_per_file in d6\FieldInstanceSettings::convertSizeUnit

Problem/Motivation

Getting this error while working on PHP8.1 compatibility in Commerce Migrate. See #3270534: Test failures with PHP 8.1.

1) Drupal\Tests\commerce_migrate_ubercart\Functional\uc6\MigrateUpgradeUbercart6Test::testMigrateUpgradeExecute
Exception: Deprecated function: strlen(): Passing null to parameter #1 ($string) of type string is deprecated
Drupal\field\Plugin\migrate\process\d6\FieldInstanceSettings->convertSizeUnit()() (Line: 80)

process\d6\FieldInstanceSettings->convertSizeUnit() is converting the max_filesize_per_file field setting which in the Ubercart database is a NULL. I then checked imagefield and it turns out that in Drupal 6 the setting max_filesize_per_file can be set to NULL so that case should be handled in core. See https://git.drupalcode.org/project/imagefield/-/blob/6.x-3.x/imagefield.....

Steps to reproduce

Proposed resolution

If max_filesize_per_file is NULL set it to an empty string.

Remaining tasks

Patch, review, commit.

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A