Fix interval constraint value normalization in contour traces

[Lexachoc]

Closes #8124

Fix incorrect normalization of contours.value for interval constraints.

This causes cases such as:

[]           -> [NaN, NaN]
[5]          -> [5, NaN]
[2,4,8,10]   -> [8,10]

This PR normalizes them as:

[]           -> [0,1]
[5]          -> [5,6]
[2,4,8,10]   -> [2,4]

The issue is in constraint_defaults.js, where the array length is checked against contours.length instead of contours.value.length, and arrays longer than two values are handled with slice(2) instead of slice(0, 2).

The relevant logic is this part:

plotly.js/src/traces/contour/constraint_defaults.js

Lines 59 to 71 in 1ef3c78

	} else if(contours.value.length > 2) {
	contours.value = contours.value.slice(2);
	} else if(contours.length === 0) {
	contours.value = [0, 1];
	} else if(contours.length < 2) {
	zvalue = parseFloat(contours.value[0]);
	contours.value = [zvalue, zvalue + 1];
	} else {
	contours.value = [
	parseFloat(contours.value[0]),
	parseFloat(contours.value[1])
	];
	}

This affects both contour and contourcarpet, since they share the same constraint defaults.