Less file not listed in sources sourcemap array if it contains only variables

If I import a less file that only includes variables then it does not get included in sources. Here's an example, notice that the map file at the end does not include variables.less as a source.

main.less

@import "variables.less";

a { color: @theme-primary; }

variables.less

@theme-primary: #2C3E50;

main.css.map

{
  "version":3,
  "sources":["/less/main.less"],
  "names":[],
  "mappings":"AAGA;EACC,…"
}

Reproduce the example using main.less, variables.less, and main.css.map, then trace the sourcemap generation for imports containing only variables. Confirm the fix by adding or updating a regression test so variables.less appears in the map's sources array without changing the generated CSS.