Fix panic of ssh public key page after deletion of auth source (#31829) (#31836)

Backport #31829 by @lunny

Fix #31730 

This PR rewrote the function `PublicKeysAreExternallyManaged` with a
simple test. The new function removed the loop to make it more readable.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/asymkey/ssh_key_test.go

@@ -12,6 +12,8 @@ import (
 	"strings"
 	"testing"
 
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
 	"code.gitea.io/gitea/modules/setting"
 
 	"github.com/42wim/sshsig"
@@ -503,3 +505,11 @@ func runErr(t *testing.T, stdin []byte, args ...string) {
 		t.Fatal("expected error")
 	}
 }
+
+func Test_PublicKeysAreExternallyManaged(t *testing.T) {
+	key1 := unittest.AssertExistsAndLoadBean(t, &PublicKey{ID: 1})
+	externals, err := PublicKeysAreExternallyManaged(db.DefaultContext, []*PublicKey{key1})
+	assert.NoError(t, err)
+	assert.Len(t, externals, 1)
+	assert.False(t, externals[0])
+}