Skip RunAndReturn() on functions with no returns

On functions that don't return anything, RunAndReturn() doesn't call
the function given. To avoid misleading users, this changes mockery to
not generate RunAndReturn() on such functions; it doesn't seem
particularly useful anyway because there's no return value to specify.

Return() is preserved even though it isn't needed either, to avoid
confusion with mock.Call.

Signed-off-by: Stephen Kitt <skitt@redhat.com>

pkg/generator.go

@@ -964,11 +964,13 @@ func (_c *{{.CallStruct}}{{ .InstantiatedTypeString }}) Return({{range .Returns.
 	_c.Call.Return({{range .Returns.Names}}{{.}},{{end}})
 	return _c
 }
+{{- if (gt (len .Returns.Params) 0)}}
 
 func (_c *{{.CallStruct}}{{ .InstantiatedTypeString }}) RunAndReturn(run func({{range .Params.Types}}{{.}},{{end}})({{range .Returns.Types}}{{.}},{{end}})) *{{.CallStruct}}{{ .InstantiatedTypeString }} {
 	_c.Call.Return(run)
 	return _c
 }
+{{- end}}
 `)
 }