Empty object coercion

[tpburch]

Addresses #166

Previously, two things were working together to create this result:

1. The coercion function would return undefined for an empty object or array value for body parameters (parameter.schema exists).
2. The result of running the coercion function ('undefined' in this case) was sent to schema.validate. Because this function is awaited, the actual value is returned, and is subsequently inspected for an error property. Since the value is undefined, this throws an unexpected error.

The fix applied is:

1. Don't coerce empty objects or arrays into undefined. Just return the empty object/array.
2. Don't await calls to validate. This is not as async function, so await is not needed. In this case, awaiting validate caused undefined to be returned rather than an object with value: undefined, which led to the unexpected error.
3. Add a check to catch cases where the result of validate is falsy for some reason, although this should not happen due to the other changes.

After applying this fix, cases where an in: body parameter is specified but not provided return a 400 with an error message resolving correctly to the parameter name.

[tlivings]

Can you add a test to validate the issue has been fixed?

[tlivings]

LGTM

[tlivings]

i imagine this was a left over from earlier version of async joi validate calls.