values.schema.json & helm docs bugs

[albertlockett]

There are some cases where generating schema.json does not work correctly:

• when a struct has an interface field which is null and a custom type mapper is supplied, the function panics trying to convert the interface field into a map[string]interface{} for the custom type mapper if the field's value is nil
• if multiple charts are generated in the same process, the choice of whether to render the schema.json can leak between chart generations
• the tag value jsonschema:"required" is not well supported on root custom values

There is also an issue where generating helm docs will panic

• if the argument that is passed is a struct with a field which contains a map where the map values are structs and those structs have fields which are pointers and has tag omitChildren:"true" and the pointer and the map is nil/empty