[Parser] Allow for pretty-printing of nodes with null parents
[Parser] Allow for pretty-printing of nodes with null parents

Nodes with null parents typically occur at a document root. These nodes
have `null` as the value for both `parent` and `env` fields.

Currently the pretty-printer for `Node` instances will ignore the
possibility that these values are `null`, leading the extension to crash
when passed such values.

heptio/vscode-jsonnet,heptio/vscode-jsonnet