improve usability of includeSources (#3057)

Exclude source contents from input source map if `includeSources=false`

fixes #3041

lib/minify.js

@@ -182,6 +182,8 @@ function minify(files, options) {
                     } else for (var name in files) if (HOP(files, name)) {
                         options.output.source_map.get().setSourceContent(name, files[name]);
                     }
+                } else {
+                    options.output.source_map.get()._sourcesContents = null;
                 }
             }
             delete options.output.ast;