* Html Writer - Do Not Generate background-color When Fill is None
For PR #3002, I noted that there was a problem with Dompdf truncating images. I raised an issue with them (https://github.com/dompdf/dompdf/issues/2980), and they agree that there is a bug; however, they also suggested a workaround, namely omitting background-color from any cells which the image overlays. That did not at first appear to be a solution which could be generalized for PhpSpreasheet. However, investigating further, I saw that Html Writer is generating background-color for all cells, even though most of them use the default Fill type None (which suggests that background-color should not be specified after all). So this PR changes HTML Writer to generate background-color only when the user has actually set Fill type to something other than None. This is not a complete workaround for the Dompdf problem - we will still see truncation if the image overlays a cell which does specify a Fill type - however, it is almost certainly good enough for most use cases.
In addition to that change, I made the generated Html a little smaller and the code a little more efficient by combining the TD and TH styles for each cell into a single declaration and calling createCssStyle only once.
* Revamp One Test
Look for both td.style and th.style instead of just td.style in test.
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