core: NewResourceConfigShimmed not to crash without schema

Although there isn't really a good reason why there should be no schema
in practice, it's better for us not to crash right now while we're still
updating all of the callers (mostly tests) to make schema available.

terraform/resource.go

@@ -2,6 +2,7 @@ package terraform
 
 import (
 	"fmt"
+	"log"
 	"reflect"
 	"sort"
 	"strconv"
@@ -256,6 +257,11 @@ func newResourceConfigShimmedComputedKeys(obj cty.Value, schema *configschema.Bl
 	var ret []string
 	ty := obj.Type()
 
+	if schema == nil {
+		log.Printf("[WARN] NewResourceConfigShimmed: can't identify computed keys because no schema is available")
+		return nil
+	}
+
 	for attrName := range schema.Attributes {
 		if !ty.HasAttribute(attrName) {
 			// Should never happen, but we'll tolerate it anyway