Add unit tests for internal/render renderFile

Add white-box tests (in render_test.go, package render) covering renderFile's
builtin template functions and edge cases: the yaml and deref functions (nil
and non-nil pointer), user-provided template funcs, the template parse-error
path, and the whitespace-only, empty-file and empty-kind skip paths. This
brings internal/render to 100% statement coverage.

Signed-off-by: Abrar Shivani <ashivani@nvidia.com>