尽管检查，null仍出现在数组中

Seeing some strange behavior coming back when I call up a list of keywords - I get my JSON list but one of the keywords is null, which turns the JSON bad:

    "west": [
  "15711989"
],
"work": [
  "15711989"
],
null: [
  "15711992"
],
"€": [
  "15711992"
],
"上水": [
  "15711170"

I started running a few checks while doing the loop, but none of them are catching it:

if (!is_numeric($keyword) && $keyword != "" && $keyword != " " && $keyword !== NULL && $keyword != "null") {

What am I doing wrong?