Empty <fill></fill> are generated in certain cases

[brc-dd]

Description

In certain cases this is being generated in styles.xml:

  <fills count="3">
    <fill>
      <patternFill patternType="none"></patternFill>
    </fill>
    <fill>
      <patternFill patternType="gray125"></patternFill>
    </fill>
    <fill></fill>
  </fills>

    <xf numFmtId="164" fontId="0" fillId="2" borderId="0" xfId="0" applyNumberFormat="true" applyFill="true" applyAlignment="false">
      <alignment></alignment>
    </xf>

Here, the last <fill></fill> is invalid and causing errors when using strict parsers like openpyxl.

Steps to reproduce the issue

It's bit hard to create a minimal reproduction. The code is part of our internal application. Though the fix is similar to #332

diff --git a/styles.go b/styles.go
index 5992cc3..36c90d0 100644
--- a/styles.go
+++ b/styles.go
@@ -1470,7 +1470,7 @@ func (f *File) extractBorders(bdr *xlsxBorder, s *xlsxStyleSheet, style *Style)
 // extractFills provides a function to extract fill styles settings by
 // given fill styles definition.
 func (f *File) extractFills(fl *xlsxFill, s *xlsxStyleSheet, style *Style) {
-	if fl != nil {
+	if fl != nil && (fl.GradientFill != nil || fl.PatternFill != nil) {
 		var fill Fill
 		if fl.GradientFill != nil {
 			fill.Type = "gradient"

This is the only place where style.Fill = fill was being assigned without any checks on whether the fill has a gradientfill/patternfill child.

Describe the results you received

<fill></fill> is being generated and fillId="2" is referencing it.

Describe the results you expected

No empty fill should be generated. fillId="0" should be used instead.

Go version

1.24.2

Excelize version or commit ID

2.9.0

Environment

OS: darwin/arm64 – macOS 15.4.1 (24E263)
Microsoft Excel Version: 16.96.1 (25042021)

Validations

• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• The provided reproduction is a minimal reproducible example of the bug.

[xuri]

Thanks for your issue. Could you show us a complete, standalone example program or reproducible demo? If you open an existing workbook, please provide the file attachment without confidential info. I'm not sure if this relates to #2014. Please also try using master branch code, upgrade to the master branch code by go get -u github.com/xuri/excelize/v2@master.

[brc-dd]

Thanks. It works with the latest commit. Sorry for the noise.

[xuri]

Thanks for your feedback.