Fix Array.prototype.concat() with sparse arrays

Fix Array.prototype.concat() with sparse arrays

Previously, the fast concat path represented a missing source property with
njs_value_invalid and still created the corresponding destination property.
For a slow sparse destination this materialized an internal hole sentinel as
a normal property, which later property access could interpret as an invalid
accessor.

The fix is to advance the destination index without creating a property when
the source lookup reports a hole.

The issue was introduced in 232fb594 (0.7.2).

This closes #1095 issue on GitHub.