Fix panic for module calls without source (#1950)

Author: wata727

terraform/loader.go

@@ -96,6 +96,10 @@ func (l *Loader) LoadConfig(dir string, callModuleType CallModuleType) (*Config,
 func (l *Loader) moduleWalkerFunc(walkLocal, walkRemote bool) ModuleWalkerFunc {
 	return func(req *ModuleRequest) (*Module, *version.Version, hcl.Diagnostics) {
 		switch source := req.SourceAddr.(type) {
+		case nil:
+			// Case for no source attribute. This is usually invalid, but is ignored to prevent panic.
+			return nil, nil, nil
+
 		case addrs.ModuleSourceLocal:
 			if !walkLocal {
 				return nil, nil, nil

terraform/test-fixtures/v0.15.0_module/module.tf

@@ -7,3 +7,5 @@ module "consul" {
    source = "hashicorp/consul/aws"
    version = "0.9.0"
 }
+
+module "no_source" {}