Fix#2432. Probably for memory reasons, PhpSpreadsheet divides its data into chunks when writing to Mpdf. However, if the first chunk has so many styles that the `body` tag is not included in the chunk, Mpdf will not handle it correctly. Code is changed to ensure that the first chunk always contains the body tag.
Because this error becomes evident only when opening the PDF file itself, it is difficult to write a test case. I have instead added a new sample file which meets the conditions which would have led to the error, and which can be examined to show that it is created correctly.
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