fix: drop empty path and unrenderable value from function call errors - #140

What this changes

FunctionCallError.Error() builds its message from three parts unconditionally, so when a part is
missing the message shows the gap instead of omitting it:

• Path is empty for a root value, which renders as function call error for : ... with nothing
  before the colon.
• NewFunctionCallError puts the formatting failure into the Value field when util.ToString
  fails, so the message ends with (value: cue/format: unsupported node type <nil>). That reads as
  though the user's value caused a CUE problem, and sends people looking for a CUE error in their own
  input that is not there.

Both clauses are now emitted only when they carry something. A message that has both parts is
unchanged.

Reported in kubevela/kubevela#7365, where it surfaces on every shared-resource conflict:

shared-cm: function call error for : Dispatch: pre-dispatch dryrun failed: Found 1 errors.
[(cannot apply ApplyOption: existing object ConfigMap default/contested-cm is managed by other
application default/owner-a)] (value: cue/format: unsupported node type <nil>)

That issue also needs a go.mod bump in kubevela/kubevela before the fix reaches users, so I have
not used a closing keyword here.

Note on Value

When the value cannot be rendered, the exported FunctionCallError.Value field is now empty rather
than holding the formatting error's text. FunctionCallError is only constructed and read inside
cue/cuex, and the existing case in compiler_test.go passes a non-empty Value, so nothing else
changes.

Testing

• go test -count=1 ./cue/cuex/ and make unit-test (full suite with envtest)
• golangci-lint run ./... with the pinned 1.60.1 from makefiles/const.mk (0 issues)
• gofmt and go vet
• The added tests fail against the current code and pass with the change. The
  NewFunctionCallError case asserts the message does not contain cue/format, which is the exact
  string from the report.

AI assistance

Claude Code helped with this change and the tests. I reviewed the result and checked the behavior
myself with the test suite and linters.

Summary by cubic

Fixes function call error messages so they omit empty paths and unrenderable values, removing misleading output like function call error for : and (value: cue/format: unsupported node type <nil>). This addresses the error messages seen in kubevela/kubevela#7365. Messages that include both a path and a value are unchanged. The exported FunctionCallError.Value field is now empty when the value can't be rendered, but only internal cue/cuex code uses it. Note: a go.mod bump in kubevela/kubevela is still needed before the fix reaches users.

^{Written for commit 1f3eff9. Summary will update on new commits.}