Fix false number recomposing and NaN number problem

[agoat]

This PR fixes two problems.

Fix One
When animating numbers of a path or other number series and the numbers may change from a positive to a negative value, like in the following example, the recomposing of the number string will be wrong.

var ani = anime({
	targets: '#gras',
	d: [
		'M226.07 433.57c1.43-75-73.93-166.07-179.64-208.2',
		'M226.07 433.57c1.43-75 .36-212.5-40.36-350.36',
		'M226.07 433.57c1.43-75 67.5-231.78 233.22-140.36',
		'M226.07 433.57c1.43-75 .36-212.5-40.36-350.36',
		'M226.07 433.57c1.43-75-73.93-166.07-179.64-208.2',
	],
	duration: 3000,
	easing: 'linear',
	loop: true,
})

The problem results in the fact, that the returned string will be recomposed from the tween.to.string array, but the shortest form in a svg image can be no space (separator) but a negative number. So the string array will have '' and when the from number is still positive there is no separator to the following number.

Here is an example of how it will look like:
https://codepen.io/anon/pen/xdvjOM

And here how it will look with my fix:
https://codepen.io/anon/pen/EmqBNv

Second fix
This will avoid the returning of NaN numbers. It will also fix the issue in #167 and maybe #119 and #164.

The problem is that if the length of the tween.from.number array is lower than the tween.to.number array, the value calculation try to calculate with a non existing array value which then results in NaN. So we have to use at least a 0 when calculating.