[SCHEMA] Fix plant_type enum "null", by changing to null (no inverted commas)

[dancasey-ie]

The plant_type enum list includes the string option "null", this should be replaced by null, excluding the inverted commas

[stephenholleran]

@dancasey-ie just to clarify you mean remove the double quotes?

[dancasey-ie]

@stephenholleran yes remove double quotes