Compiling empty less file causes the map to contain string "undefined"

[OJezu]

The following:
lessc empty.less --source-map=test.map out.css
where "empty.less" is, well, empty (0 bytes) causes test.map to have following content:

undefined

Same happens if "empty.less" produces no css, e.g. has only variable definitions.
Possibly related: #1491 and gulp-community/gulp-less#121 - very strange error that comes from following line:
return new Buffer(str).toString('base64')
from lib/less-node/environment.js
When str === undefined new Buffer throws. At first glance seems that the reason str is undefined is the same as that "undefined" gets written to map file.

Less version: less@2.3.1

[yatskevich]

Looks like there is a regression in 2.5.0.

root@ba0c79b2ec1b:~# nodejs --version
v0.10.25
root@ba0c79b2ec1b:~# npm --version
1.3.10
root@ba0c79b2ec1b:~# lessc --version
lessc 2.5.0 (Less Compiler) [JavaScript]
root@ba0c79b2ec1b:~# lessc empty.less --source-map=empty.css.map empty.css

empty.css.map contains undefined.