Fix for 1248 - Corrected handling of section layout FlowLayout/Vertical in provisioning

[Magnus-K-work]

Related issue:
#1248 - Template provisioning can't handle section layouts or published state?

What is changed:

• ClientSidePageContentsHelper.cs - lines 269-273 Uses correct layout type for FlexibleLayoutSection and FlexibleLayoutVerticalSection
• ClientSidePageContentsHelper.cs - line 203, re-throw correctly to keep stacktrace and inner exceptions
• TemplateListInstancesWriter.cs - line 127-129, handle null value for xmlField.Attribute("Required"), as this was causing null point exceptions in test runs

Tests:

• Not run in full, as many provisioning tests require installation of O365 Starter Intranet which is outdated/archived

Second public PR ever, please help me improve. :)

BR
Magnus

[Magnus-K-work]

I'll keep these PRs alive by merging up from PnP.Framework / Dev for each release.