FIX Don't error if template global is null

[GuySartorelli]

Description

Fixes a bug where if template variables from a TemplateGlobalProvider return null and you chain off them, you'd get a null reference error.

The getObj() method is only ever called in one place, which sets the current item in scope - so if the current item in scope is null, we just keep null as the item in scope, which results in the behaviour we expect (i.e. nulls result in just not dumping anything to the template, are falsy in if statements, etc).

Manual testing steps

See reproduction steps in the issue description

Issues

• Chained properties result in errors if TemplateGlobalProvider returns null #11330

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[GuySartorelli]

I wanted to use : null as a typehint here but apparently PHP doesn't allow that 😅

[NightJar]

isset(null) === false && isnull(null) === true.
PHP doesn't have a concept of "is set, but is null", so I guess that makes sense 😅

[GuySartorelli]

Ran this before doing the code change and can confirm it fails by throwing an exception without the code change.