Better error for writeFile where obj is undefined

[RyanZim]

node-jsonfile/utils.js

Lines 3 to 5 in 674dbeb

	const str = JSON.stringify(obj, replacer, spaces)
	return str.replace(/\n/g, EOL) + EOF

JSON.stringify() can return undefined when obj is undefined (or a bare function, or a Symbol), which would cause str.replace(...) to throw TypeError: Cannot read properties of undefined. We should add some basic type-checking here, and throw with a more intelligent error message for these cases.