Filter falsey _npmUser values

Some packages that were published with much older versions of npm (e.g., 1.0.x) don't have `maintainers` or `_npmUser` fields in their package.json files. This triggers a client-side error when doing `npm search` because the maintainers field will be `[null]` and the npm client attempts to read the `name` field on a null value.

This patch filters falsey `_npmUser` values, which prevents the client crash.