fix: Token.ParseIssueReference crashing on empty string (#8260)

A fix for a bug introduced by me earlier, where attempting to parse an issue reference in an empty token would crash.

An empty token occurs if the search string is `\` or `"` (among other scenarios, probably). I'll make another PR that avoids having empty tokens (seems like a good idea).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8260
Reviewed-by: Shiny Nematoda <snematoda@noreply.codeberg.org>
Co-authored-by: Danko Aleksejevs <danko@very.lv>
Co-committed-by: Danko Aleksejevs <danko@very.lv>

modules/indexer/issues/internal/qstring.go

@@ -25,7 +25,7 @@ type Token struct {
 
 func (tk *Token) ParseIssueReference() (int64, error) {
 	term := tk.Term
-	if term[0] == '#' || term[0] == '!' {
+	if len(term) > 1 && (term[0] == '#' || term[0] == '!') {
 		term = term[1:]
 	}
 	return strconv.ParseInt(term, 10, 64)

modules/indexer/issues/internal/qstring_test.go

@@ -169,3 +169,35 @@ func TestIssueQueryString(t *testing.T) {
 		})
 	}
 }
+
+func TestToken_ParseIssueReference(t *testing.T) {
+	var tk Token
+	{
+		tk.Term = "123"
+		id, err := tk.ParseIssueReference()
+		require.NoError(t, err)
+		assert.Equal(t, int64(123), id)
+	}
+	{
+		tk.Term = "#123"
+		id, err := tk.ParseIssueReference()
+		require.NoError(t, err)
+		assert.Equal(t, int64(123), id)
+	}
+	{
+		tk.Term = "!123"
+		id, err := tk.ParseIssueReference()
+		require.NoError(t, err)
+		assert.Equal(t, int64(123), id)
+	}
+	{
+		tk.Term = "text"
+		_, err := tk.ParseIssueReference()
+		require.Error(t, err)
+	}
+	{
+		tk.Term = ""
+		_, err := tk.ParseIssueReference()
+		require.Error(t, err)
+	}
+}