Don't confuse empty configuration variables with booleans

Config was confusing the following two variables when writing the
file back to text format:

  [my]
    empty =
    enabled

When parsed, we say that my.empty has 1 value, null, and my.enabled
is an empty string value that in boolean context should be evaluated
as true.

Saving this configuration file back to text format was ignoring the
null value for my.empty, producing a completely different file than
what Config read:

  [my]
    empty
    enabled

Instead handle the writing differently to ensure the original format
is output.  New tests cases cover the expected behavior and return
values from accessor methods.

Change-Id: Id37379ce20cb27e3330923cf989444dd9f2bdd96
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>