AbstractContextual: ignore setContext(null) calls

In scijava-common 2.0.1, a NullPointerException would be thrown.
However, in scijava-common 2.0.0, doing this was legal and had no
effect. This commit restores the former behavior (no effect), so that
scifio 0.4.1 can work with newer scijava-common versions as expected.

One consequence of this fix is that the previous bugfix,
d4c185e8eb1d4e483f88c1e3f13e1a109962bcb1, is now redundant
and thus has been reverted.