Made two small changes to the SQL NULL lesson.

There was a small mistake in the description of the penultimate
example: the text said that we wanted to exclude measurements taken by
Dyer, whereas the SQL query was excluding measurements taken by Lake.
This was corrected.

Also, I added a brief paragraph on the effects of NULL values on
aggregation functions, referring the next section and slightly modified
a line that was saying "null values cause headaches wherever they
appear" to say that "null values CAN cause headaches…"
Both changes to try to underline that NULL values are not a bad thing.
They require extra caution, but are helpful.

Changes were made to the iPython notebook (05-null.ipynb), which was compiled to make a
new Markdown file (05-null.md).

This is my first pull request to the repository, and part of my Software Carpentry instructor training, so apologies if I'm doing it wrong :)