[JENKINS-74867] Fix cloning entire parent folder when 'copy from' field is cleared after inputs

[Lmh-java]

See JENKINS-74867.

Testing done

• smoke-test
• manual test, cannot reproduce the bug anymore.

Proposed changelog entries

Besides when retrieving the item to be copied by string, not only check if the path is null, but we should also check if the string is empty.

public Item getItem(String pathName, ItemGroup context) {
        if (context == null)  context = this;
        // check if pathName is "" as well
        if (Util.fixEmpty(pathName) == null) return null;

        if (pathName.startsWith("/"))   // absolute
            return getItemByFullName(pathName);

Why this bug occurs:
The backend reads in a "" (empty string) when user clears the 'copy from' field, which leads to getItem() function to return the parent directly. As a result, the parent of the current folder will be copied.

After the change, if the user tries to submit a new item when both 'job type' and 'copy from' are unset, the user will be prompted to specify which item to copy from. (as shown below)

Proposed upgrade guidelines

N/A

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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

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PS: after this PR, I would do more work on this:

• Improve UX in frontend. E.g. provide a radio button to allow the user to explicitly express their intentions to copy from existing items.