empty variables not shown in `\raw`

[kenshaw]

Empty variables should show in \raw.

For example, currently usql produces the following output:

(not connected)=> \set a ''
(not connected)=> select :a
(not connected)-> \p
select 
(not connected)-> \raw
select 
(not connected)->

However, on the \raw above, it should have shown:

(not connected)-> \raw
select :a
(not connected)->

Comparatively:

(not connected)=> \set a b
(not connected)=> select :a
(not connected)-> \p
select b
(not connected)-> \raw
select :a
(not connected)->

And, it behaves as expected for quoted variables:

(not connected)=> \set a ''
(not connected)=> select :'a'
(not connected)-> \p
select ''
(not connected)-> \raw
select :'a'
(not connected)->

It's worth noting this is not dependent on the length of the variable name:

(not connected)=> select :aa
(not connected)-> \p
select 
(not connected)-> \raw
select 
(not connected)-> 

The reason for this behavior is partly due to the original way that variables were parsed and interpolated (at time of entry). This will require a slight overhaul to how variables are captured and interpolated.

[nineinchnick]

This is fixed by f6882c0 and c88eb8d, right?