Unable to set empty XML field i.e. <somefield/> without a value or without xsi:null=true

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# Description/Steps to reproduce

The xmlHandler equality operator "==" evaluates undefined and null values as true as follows:

xmlHandler.js, Lines 165-173
```

if (val == null) {
if (descriptor.isNillable) {
// Set xsi:nil = true
declareNamespace(nsContext, element, 'xsi', helper.namespaces.xsi);
if (typeof element.attribute === 'function') {
element.attribute('xsi:nil', true);
}
}
}

```

It would be preferable to set the xsi:nil = true only for null values and not undefined values.
If we exclude undefined values from the block above undefined values are shown in the XML as follows:

Suggest using the type safe equality operator so it only evaluates null as true:
```

if (val === null) {
if (descriptor.isNillable) {
// Set xsi:nil = true
declareNamespace(nsContext, element, 'xsi', helper.namespaces.xsi);
if (typeof element.attribute === 'function') {
element.attribute('xsi:nil', true);
}
}
}

```

The new code still allows strong-soap to set a xsi:nil=true when null value is passed, for undefined values however it will not do this.
Seems like a fairly major impact if users are used to xsi:null=true being set if they pass through undefined currently, thoughts on this?

# Link to reproduction sandbox

# Expected result

# Additional information

Start in xmlHandler.js around lines 165-173 and trace how undefined and null values are serialized. Verify that null still produces xsi:nil=true while undefined produces a self-closing element such as ; completion is confirmed when the two cases no longer share the same behavior.

Written by the indexing model from the issue text.