throw on reading undefined file path

[brendankenny]

Fixes building lighthouse in Node 15.

static-module calls brfs when it encounters a fs.readFileSync (and the other read methods) with a string file if the path can be statically determined or undefined if the path is too dynamic to figure out.

Previous to Node 15, when brfs then does fs.createReadStream(file), if file is undefined it would immediately throw, static-module would catch the error, and the original code would be retained instead of replacing it with an inline string.

Node 15 made a change to stream construction (nodejs/node#29656) that made stream construction lazier, and so the error comes later (when piping begins), static-module no longer catches the error, and everything fails instead of continuing.

This is probably the simplest fix, still throwing if file is undefined, just explicitly now instead of relying on fs.createReadStream throwing.