[ivy] 'ngClass' + 'dynamic [class] = undefined | null' completely clears out 'static class binding'

[Gretam11]

🐞 bug report

Affected Package

The issue is caused by package @angular/core or @angular/common 9.0.0

Is this a regression?

It works fine when Ivy is opted-out.

Description

I'm not sure whether this is a real bug, but it's indeed behavior which some libraries depend on. If the static class binding is specified with NgClass directive and with dynamic class binding equals null or undefined, all values from static class binding are being cleared out.
In case if any string (even empty) is specified in dynamic binding, it just merges all classes as expected.

🔬 Minimal Reproduction

I couldn't turn on ivy on stack-blitz, so here is the link on ng-run of minimal reproduction:
https://ng-run.com/edit/LKHMmMPIRiZmgFFqMt15
It's handy to turn on/off Ivy there.

🌍 Your Environment

Angular Version:

Angular CLI: 9.0.1
Node: 12.13.0
OS: win32 x64

Angular: 9.0.0
... animations, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router
Ivy Workspace: Yes

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.900.1
@angular-devkit/build-angular     0.900.1
@angular-devkit/build-optimizer   0.900.1
@angular-devkit/build-webpack     0.900.1
@angular-devkit/core              9.0.1
@angular-devkit/schematics        9.0.1
@angular/cli                      9.0.1
@ngtools/webpack                  9.0.1
@schematics/angular               9.0.1
@schematics/update                0.900.1
rxjs                              6.5.4
typescript                        3.7.5
webpack                           4.41.2

[cagataycivici]

Thank you, it was a showstopper for us to release PrimeNG 9.0.0

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