Bug report or Feature request?
Feature request

Version (complete output of terser -V or specific git commit)
4.6.6

Complete CLI command or minify() options used

terser -c --define process.env={} a.js

terser input

if (process.env.DEBUG) console.log('please remove');

terser output or error

({}).DEBUG&&console.log("please remove");

Expected result

void 0;

Discussion

The current behavior is correct from a conservative standpoint, because it's technically possible that Object.prototype.DEBUG has been defined. But given the exceedingly insignificant chance of this actually happening, it makes sense to provide the option to more aggressively inline the missing property as undefined, which would allow removing the log statement. Rollup actually does this by default already.

EDIT: clarified discussion slightly