[sr-dev] test if $var has been give a value

[Juha Heinanen]

it would be nice if someohe who knows could summarize, how to test if
these have a value assigned to them:

- $var
- $pseudo_variable (for example, message item)
- $avp
- @select
- htable entry
- what else there is that can be assigned a value

it would be VERY desirable that one way to do the test would work for
all of above, before the first release of sr comes out.

-- juha