Avoid running after/enrich without a db effect

[frenata]

Previously both standard interceptors acted upon the db coeffect if
no effect was available. Per #453 and related discussion, there is little
reason in general to do this, since this context means that the app-db
should not change.

Both functions are changed to explicitly check for the presence of the
db effect. If it is not present in the context, the interceptor
function is not run.

The previous functionality around nil and false values for the db
effect is preserved: the interceptors are run in this case. Tests are
added where needed to verify the behavior of both functions in the case
of non-present, falsey and truthy values.

The relevant docstrings are tweaked to accommodate this change.

Closes #453.

[superstructor]

Having read the prior discussion in #453, #447 and #450 and reviewed the code I am satisfied this change implements an agreed change in behaviour so merging. Sorry it took awhile to get to this and thanks for the PR @frenata !

[mike-thompson-day8]

@superstructor We need to back this out and release a new version. This is a breaking change. Some working code of ours stopped working on the upgrade from 10.6 to 10.7.

If anyone wants this feature of not running the interceptor when there's no :db effect, it should be introduced as a different interceptor with a different name (enrich2?) or something. We can't break existing code.

[mike-thompson-day8]

@frenata
I'm really sorry we had to back this out after you put the effort in, and got this PR perfect.
It is my fault that this has happened. I should have recognised the problems with this change before you went to the effort. I just didn't think through the implications of the breakage correctly.

The best approach from here is to leave the existing enrich and after interceptors unchanged, and to add two new interceptors enrich2 and after2 (probably not their final names) which have the alternative behaviour. There is a valid case for both approaches.

[frenata]

@mike-thompson-day8 No big deal, these things happen!

I'm happy to help with further work in this area if that would be helpful, but otherwise am happy to have had a chance to dig around in the re-frame internals. It's a great project that I was happy to have used at work and I was interested in giving back in some small way.

[superstructor]

This has been reverted in 0.10.8. Sorry for the inconvenience. I misjudged the change as low risk.