Handle non-reactive nulls (#231)

[coolhome]

Fix for #231 - Ensure non reactive nulls are supported.

// 'this' cannot be set to null.
Object.hasOwnProperty.call(null, ''); // TypeError Cannot convert undefined or null to object

// Not an Object...
(null).hasOwnProperty('') // TypeError: Cannot read property 'hasOwnProperty' of null

Solution would be to check if the obj is null.

[haoqunjiang]

Better do the obj !== null test in the isReactive function.
And change the obj: Object to obj: object, to make the code clearer.

[coolhome]

@sodatea thanks for the feedback!

[antfu]

This is covered by #311 and no longer an issue in v0.6.1. However, I think we can take the unit-tests.

Resolved