Removed extra blank page on print all plots

[MatthewBowles]

Fixed an issue when using File->Print All Plots where an extra blank page would be printed.

To test:

• Create/load some workspaces
• Plot multiple spectra plots
• File > Print All Plots
• Select Print To File (PDF)
• Print
• Open the file and check there are no blank pages

Fixes #11606

Release Notes

Does not need to be in the release notes.

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Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

Does everything look good? Mark the review as Approve. A member of @mantidproject/gatekeepers will take care of it.

[PranavBahuguna]

Code looks good and changes work as expected 👍

once checks pass