`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.<anonymous> (/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)