fix(ivy): error for empty bindings on ng-template

[crisbeto]

Fixes Ivy throwing an error if it runs into an empty property binding on an ng-template (e.g. <ng-template [something]=""></ng-template>) by not generating an update instruction for it.

Fixes #30801.
This PR resoves FW-1356.

[AndrewKushnir]

Presubmit

[Airblader]

Based on @JoostK's comment I was expecting that the fix would be in the visit method of EmptyExpr to not return undefined(?)

[JoostK]

@Airblader Changing EmptyExpr.visit not to return undefined would mean that we'd need to return something else. Since the actually correct behavior (at least according to what VE has always done) would be that should be that no binding code should be generated at all, easiest is to just deal with the undefined value.

Going forward, it seems like a good idea to either have type-safety regarding the potential undefined value here, or the template parser should be changed to always error on an empty binding. Then, EmptyExpr.visit could simply throw, as it should be an illegal operation to compile a template containing EmptyExpr nodes. FYI: there's 6 places where an EmptyExpr could be created during parsing, 5 of which do produce an error. Only the empty property binding has slipped through.

From a bugfix perspective, this fix looks adequate to me.

[crisbeto]

Thank you for the feedback @JoostK. Setting back the merge ready label since I've updated it to address the comment.

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