Mapped empty string to undefined config.
Prevented undefined GDAL_DATA to fail if env has it.
Removed "null" as uninitialized value.
Reviewed poorly designed config validation.

Undefined config vars could just as well me missing from the config list that
couch_config:get/1 returns. On the other hand I don't want to keep track of
that list myself, so I've let get_config throw the code 500 responses.
Moved command generation into get_command/1.
Started to write GET /_ogr2ogr into a plugin test.

At the end of the day, the response should say if either the command is wrong
(because quotation marks are missing or something), or if GDAL_DATA is wrong or
if fallback_crs is wrong.
Removed a trailing whitespace.
Inserted colon before config name literal.
Moved test procedure into local function Test/1.
Added tests for GDAL_DATA and fallback_crs.
Apparently this is valid and it looks better somehow.