Don't assume a user has defined `paths`

[enocom]

In deps.edn, it's valid to leave off a paths key and the tool assumes src. This library should do the same.

See https://github.com/StediInc/lambda/blob/afa61b28a281af09de389397b0344e693a5defb9/src/stedi/lambda/build.clj#L69