Add nil checks for schema when evaluating XRD schema field

[tnthornton]

Description of your changes

XRDs do not have a hard requirement that a version has a schema. Given that, the previous implementation assumed that there would be a schema object on every version; which results in a panic when a schema does not exist.

This can also be seen if an end user is typing an XRD from scratch and has not yet defined the schema.

Fixes #160

I have:

• Read and followed Upbound's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

1. Verified that a panic is no longer seen in xpls when using the https://github.com/danielinclouds/play_crossplane_plugin repo.
2. Verified that typing up the schema and partially supplying its definition does not result in a panic. Effectively mocking the authoring flow of typing everything individually.

[hasheddan]

Thanks @tnthornton!