Avoid unnecessary 500 panic when a commit doesn't exist (#28719)

In #26851, it assumed that `Commit` always exists when
`PageIsDiff==true`.

But for a 404 page, the `Commit` doesn't exist, so the following code
would cause panic because nil value can't be passed as string parameter
to `IsMultilineCommitMessage(string)` (or the StringUtils.Cut in later
PRs)

templates/base/head_opengraph.tmpl

@@ -17,7 +17,7 @@
 	{{else if or .PageIsDiff .IsViewFile}}
 		<meta property="og:title" content="{{.Title}}">
 		<meta property="og:url" content="{{AppUrl}}{{.Link}}">
-		{{if .PageIsDiff}}
+		{{if and .PageIsDiff .Commit}}
 			{{- $commitMessageParts := StringUtils.Cut .Commit.Message "\n" -}}
 			{{- $commitMessageBody := index $commitMessageParts 1 -}}
 			{{- if $commitMessageBody -}}

tests/integration/repo_test.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
 	"code.gitea.io/gitea/tests"
 
 	"github.com/PuerkitoBio/goquery"
@@ -448,3 +449,12 @@ func TestGeneratedSourceLink(t *testing.T) {
 		assert.Equal(t, "/user27/repo49/src/commit/aacbdfe9e1c4b47f60abe81849045fa4e96f1d75/test/test.txt", dataURL)
 	})
 }
+
+func TestViewCommit(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	req := NewRequest(t, "GET", "/user2/repo1/commit/0123456789012345678901234567890123456789")
+	req.Header.Add("Accept", "text/html")
+	resp := MakeRequest(t, req, http.StatusNotFound)
+	assert.True(t, test.IsNormalPageCompleted(resp.Body.String()), "non-existing commit should render 404 page")
+}