Create index(es) for optional named parameters in init() method

[amulet1]

There is a possibility that some named parameters for init() would be omitted when passed to addVariable()/addHidden()/insertVariableBefore().

Suppose we have this:

function init(...$params) {
   $parm1 = $params[0];
   $parm2 = $params[1];
   ...
}

Calling addVariable() with $params = [ 'parm1' => 'abc', 'parm2' => 123 ] results in init(...$params) called with $params = [ 0 => 'abc', 1 => 123 ], which is correct.

But if parm1 is omitted, i.e. $params = [ 'parm2' => 123 ], then init(...$params) would be called with $params = [ 1 => 123 ]. However PHP converts this internally to $params = [ 0 => 123 ], which results in the wrong parameter assignment - parm1 (not parm2) gets assigned 123.

This PR addresses handling of omitted parameters (we do not seem to have cases like that though) by making sure indexes for such parameters exist.

With this PR the params in the previous example becomes [ 0 => null, 1 => 123 ], which fixes the issue.