Detect when variables are declared and not set

It is possible to declare a variable but not assign a value to it using
`export x`. When you do this, `declare -p x` shows the variable but does
not show an "=" nor any value afterwards.

When running another command, this variable will not be added to the
environment variables even though it is flagged as exported. Most likely
it's because the value is not a string and can't be easily converted to
a string; this is the same behavior as arrays.

Using `[[ -v x ]]` is also inadequate and I believe its because there
are false positives when trying to access data, which goes on to break
tests and the new braces and parenthesis indirection. Perhaps it could
be reviewed and made to work.

The best solution so far is to combine `declare -p` with `[[ -v` to see
if the variable is declared and if a value is set.

Closes #75.

tests/help

@@ -94,7 +94,7 @@ This is open source!  Please feel free to contribute.
 
 https://github.com/tests-always-included/mo
 
-MO_VERSION=3.0.6
+MO_VERSION=3.0.7
 EOF
 }