Fix formatting a lambda expression containing an extension method

[vanya-lebedev]

Problem

Calling an extension method from the expression provided to Satisfy causes an exception. Solves #1688

Solution

Ignore null expressions in the constant visitor.

Not in scope

User-friendly formatting of an external lambda expression parameter is not solved as part of this pull request. I can see that it is also not yet cleanly solved in other APIs.
E.g. Should().Contain(a => allowed.Contains(a)) would fail with the following error message:

"Expected actual {4} to have an item matching value(FluentAssertions.Specs.Formatting.PredicateLambdaExpressionValueFormatterSpecs+<>c__DisplayClass5_0).allowed.Contains(i).";

IMPORTANT

• The code complies with the Coding Guidelines for C#.
• The changes are covered by a new or existing set of unit tests which follow the Arrange-Act-Assert syntax such as is used in this example.
• If the contribution adds a feature or fixes a bug, please update the release notes, which are published on the website.
• If the contribution changes the public API the changes needs to be included by running AcceptApiChanges.ps1/AcceptApiChanges.sh.
• If the contribution affects the documentation, please include your changes in this pull request so the documentation will appear on the website.

[vanya-lebedev]

hmm... can't figure out how to link this PR to the issue. The linked issues section is disabled for me....

[dennisdoomen]

hmm... can't figure out how to link this PR to the issue. The linked issues section is disabled for me....

Normally you just add a phrase like closes #1234