definition.json requires a "variable" block or spk infra generate will fail

[yradsmikham]

Describe the bug:
In a multi-cluster set-up, if the parent or root definition.json file does not specify a "variable" block then spk infra generate will fail with There was an error with generating the spk tfvars object.

To Reproduce:
Use a parent definition.json without a variable block.

Expected behavior:
spk infra generate should be handle generation even though there are no "shared" variables in the parent definition.json.

Does this require updates to documentation?:
No.