Array with null elements crashes program

`xml({ foo: null })` works fine, but `xml({ bar: [ null, null ] })` causes the program to crash. I know it seems like an edge case, but I am converting a JSON document from a database query and some documents contain arrays with null elements.

Here is the stack trace:

> events.js:160
> throw er; // Unhandled 'error' event
> ^
> TypeError: Cannot convert undefined or null to object
> at /x/node_modules/xml/lib/xml.js:185:44
> at Array.forEach (native)
> at resolve (/x/node_modules/xml/lib/xml.js:183:24)
> at add (/x/node_modules/xml/lib/xml.js:47:24)
> at xml (/x/node_modules/xml/lib/xml.js:89:9)
> at [REDACTED]
> at Query. (/x/node_modules/mongoose/lib/model.js:3340:16)
> at /x/node_modules/kareem/index.js:259:21
> at /x/node_modules/kareem/index.js:127:16
> at _combinedTickCallback (internal/process/next_tick.js:67:7)
> at process._tickCallback (internal/process/next_tick.js:98:9)