Add a missing assert in constraint resolver

[am11]

In other case (few lines below), we have this assert before assigning this nullable ptr a value.

[am11]

cc @trylek, this assert makes SVM with contravariant issue (#78621) a bit more clear.

command:

$ CORE_ROOT=$(pwd)/artifacts/bin/testhost/net8.0-osx-Debug-arm64/shared/Microsoft.NETCore.App/8.0.0 \
    artifacts/bin/testhost/net8.0-osx-Debug-arm64/shared/Microsoft.NETCore.App/8.0.0/corerun \
    ../svm-contravariance/bin/Debug/net8.0/svm-contravariance.dll

before:

zsh: segmentation fault  CORE_ROOT=  ../svm-contravariance/bin/Debug/net8.0/svm-contravariance.dll

after:

Assert failure(PID 58618 [0x0000e4fa], Thread: 29822069 [0x1c70c75]): pfForceUseRuntimeLookup != NULL
    File: /Users/am11/projects/runtime/src/coreclr/vm/methodtable.cpp Line: 8515
    Image: /Users/am11/projects/runtime/artifacts/bin/testhost/net8.0-osx-Debug-arm64/shared/Microsoft.NETCore.App/8.0.0/corerun

zsh: abort      CORE_ROOT=  ../svm-contravariance/bin/Debug/net8.0/svm-contravariance.dll