client-gen: avoid panic for empty groups

staging/src/k8s.io/code-generator/cmd/client-gen/generators/fake/generator_fake_for_group.go

@@ -45,7 +45,7 @@ var _ generator.Generator = &genFakeForGroup{}
 
 // We only want to call GenerateType() once per group.
 func (g *genFakeForGroup) Filter(c *generator.Context, t *types.Type) bool {
-	return t == g.types[0]
+	return len(g.types) == 0 || t == g.types[0]
 }
 
 func (g *genFakeForGroup) Namers(c *generator.Context) namer.NameSystems {

staging/src/k8s.io/code-generator/cmd/client-gen/generators/generator_for_expansion.go

@@ -36,7 +36,7 @@ type genExpansion struct {
 
 // We only want to call GenerateType() once per group.
 func (g *genExpansion) Filter(c *generator.Context, t *types.Type) bool {
-	return t == g.types[0]
+	return len(g.types) == 0 || t == g.types[0]
 }
 
 func (g *genExpansion) GenerateType(c *generator.Context, t *types.Type, w io.Writer) error {

staging/src/k8s.io/code-generator/cmd/client-gen/generators/generator_for_group.go

@@ -46,7 +46,7 @@ var _ generator.Generator = &genGroup{}
 
 // We only want to call GenerateType() once per group.
 func (g *genGroup) Filter(c *generator.Context, t *types.Type) bool {
-	return t == g.types[0]
+	return len(g.types) == 0 || t == g.types[0]
 }
 
 func (g *genGroup) Namers(c *generator.Context) namer.NameSystems {