Consider adding nil guards in intermediate models

[fmartingr]

Rant: Not sure if idiomatic or not, but I wonder if we should start considering nil in all these util functions around the project.

For example, this is completely valid code:

var foo *SlackPost = nil;
if (foo.IsPlainMessage()) {
    ...

But it will create a panic in execution.

If we were to add a line here like:

if p == nil {
    return false
}

There will be no panic in execution.

There are people that would claim that a panic in execution is fine, since it means we are using the function wrong, and it is better to "fail soon and fail hard".

Others may claim that a panic in execution cannot be accepted.

But then in many places (not the case here from what I saw) we have checks like foo != nil && foo.IsPlainMessage(). So I wonder if it is a pattern we should start using.

Originally posted by @larkox in #75 (comment)