fix: prevent NPE in afterWorkbookDispose

[csbiy]

Purpose of the pull request

Fixes #788

The afterWorkbookDispose method throws NullPointerException when hasBeenInitializedSheetIndexMap contains null values.

What's changed?

Added a guard clause to skip null entries in the map before accessing getSheet().

Before:

for (WriteSheetHolder writeSheetHolder : writeSheetHolderMap.values()) {
    if (writeSheetHolder.getSheet() == null  // NPE here when writeSheetHolder is null
            || !(writeSheetHolder.getSheet() instanceof SXSSFSheet)) {
        continue;
    }

After:

for (WriteSheetHolder writeSheetHolder : writeSheetHolderMap.values()) {
    // map can contain null entries
    if (writeSheetHolder == null) {
        continue;
    }
    if (writeSheetHolder.getSheet() == null
            || !(writeSheetHolder.getSheet() instanceof SXSSFSheet)) {
        continue;
    }

Checklist

• I have read the Contributor Guide (https://fesod.apache.org/community/contribution/).
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[delei]

Hi, @csbiy Welcome!

BTW, Could you please add th code for the unit test?

[csbiy]

Hi, @delei thanks for the welcome!
I've added the unit test code as requested.
Please let me know if you’d like me to make any changes.

[delei]

LGTM