selectiveNotDeepExtend throws if property is null in extended object

[yehiyam]

When a source property is null and the target property is an object selectiveNotDeepExtend throws an exception:

Cannot read property 'constructor' of null at

node_modules/vis-util/peer/umd/vis-util.js:7108
        if (a[prop].constructor === Object) 

short reproduction:

const visutil = require('vis-util')
const a = {
    color: 'red',
    nullProp: null
}

const b = {
    color: 'blue',
    nullProp: {
        type: 'foo'
    }
}

visutil.selectiveNotDeepExtend([],a,b)

fixed by #844