`meanBy` returning `NaN` on empty lists is error prone

[tran-simon]

_.meanBy([], 'test') will return NaN

This behavior is not currently documented, meaning that users might not know that it could return NaN.

Possible solutions:

• Make meanBy return undefined instead of NaN, meaning that users using TypeScript will have to handle that case
• Improve the documentation to warn users that they mustn't assume that the returned value is a valid number