Sphere Fix: Handle empty cases in Sphere.union

[gkjohnson]

Related issue: #24694

Description

Handle cases where the spheres are empty in Sphere.union. Here is the behavior before and after.

s1 = new THREE.Sphere();
s2 = new THREE.Sphere();

s1.center.set( 1, 0, 0 );
s1.radius = 0.1;
s2.makeEmpty();

s1.union( s2 );
console.log( s1.center, s1.radius );
// BEFORE: "( 0.050000000000000044, 0, 0 )" "1.05"
// AFTER: "( 1, 0, 0 )" "0.1"

s1.makeEmpty();
s2.center.set( 1, 0, 0 );
s2.radius = 0.1;

s1.union( s2 );
console.log( s1.center, s1.radius );
// BEFORE: "( 1, 0, 0 )" "0.10000000000000003"
// AFTER: "( 1, 0, 0 )" "0.1"

[mrdoob]

@WestLangley Merging this for now. We can revert if you find any issues.

[mrdoob]

Thanks!