[main] Fix strict-mode break

[dougbu]

• use Get-Member instead of assuming $null property value
• fixes build break I caused in 13040ff

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation

Tested fix locally in the dotnet/aspnetcore repo

[dougbu]

/fyi @rainersigwald @ryalanms

[dougbu]

Question for all: How did 13040ff get past the CI❔ The build was green for both #7559 and #7558 even though taking the change into dotnet/wpf failed in dotnet/wpf#4750

[dougbu]

OIC the break wouldn't be detected unless part of the build required desktop msbuild from VS. Does this repo or dotnet/arcade-validation contain any such tests❔ If not, should one of them❔

[riarenas]

We exercise the desktop msbuild path with the usage of the signcheck tool during the signing validation job: IE. https://dev.azure.com/dnceng/internal/_build/results?buildId=1209416&view=logs&j=b11b921d-8982-5bb3-754b-b114d42fd804&t=cdcedd1f-8008-523f-9da1-cc35fbfef9a3, but there are no explicit tests for this, and clearly there's a hole since this PR had to be fix things.

[dougbu]

clearly there's a hole since this PR had to be fix things

Agreed. That build step used an xcopy msbuild and might not have exercised the codez I changed.