A lot of easily fixed problems throughout Writer/Xlsx/*, mostly supplying int rather than string as input to WriteAttribute/WriteElement. There are, in fact, so many opportunities, that I will split it over 2 or 3 PRs. But this first one will get Phpstan baseline down to the goal on its own.
Some of the other problems are also easily fixed. In particular, the docBlocks in Style/ConditionalFormatting/ConditionalDataBar do not allow for null values, and should.
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