do not modify smashed input, i.e. do not munch empty lines

pass along empty lines when smashing the input files: this ensures that input is not altered during the smash process (except of course the expansion of the import() statements)

(passed all tests: npm test)

Note: I did this as I like the d3.js output much better when it has the exact same whitespace  as the original files - this assists readability of the smashed produce, while it does not impact the minification process for those who want a minified variant.