Fix env config parsing for "GITEA____APP_NAME" (#26001)

Regression of #24832 

Fix the bug and add a test for it

Co-authored-by: Giteabot <teabot@gitea.io>

modules/setting/config_env.go

@@ -86,7 +86,7 @@ func decodeEnvSectionKey(encoded string) (ok bool, section, key string) {
 		key += remaining
 	}
 	section = strings.ToLower(section)
-	ok = section != "" && key != ""
+	ok = key != ""
 	if !ok {
 		section = ""
 		key = ""

modules/setting/config_env_test.go

@@ -48,6 +48,12 @@ func TestDecodeEnvironmentKey(t *testing.T) {
 	assert.Equal(t, "", key)
 	assert.False(t, file)
 
+	ok, section, key, file = decodeEnvironmentKey(prefix, suffix, "GITEA____KEY")
+	assert.True(t, ok)
+	assert.Equal(t, "", section)
+	assert.Equal(t, "KEY", key)
+	assert.False(t, file)
+
 	ok, section, key, file = decodeEnvironmentKey(prefix, suffix, "GITEA__SEC__KEY")
 	assert.True(t, ok)
 	assert.Equal(t, "sec", section)