This is a leftover Scrutinizer change, but it needed more attention than most others. Chart/Title DocBlocks define caption as `null|string`. However, in the wild, Excel usually presents the caption as an array, and not an array of strings but rather of RichText items. I am not sure why an array is needed since a RichText item can contain many text runs, but things are what they are.
Reader/Xlsx/ChartTitleTest reads a spreadsheet with the captions stored as a RichText array. Since it performs array operations on something the DocBlock says cannot be an array, Scrutinizer objects, although not seriously enough to fail the module. Phpstan also objects; its objection is silenced with an annotation. Aside from this test, there are other tests which do set the caption to a string, and Excel seems to handle that without a problem. So, I have changed the DocBlock to specify `array|RichText|String`. I have dropped null as a possibility; nullstring will do equally well.
Because getCaption can now return multiple datatypes, I think a new function which can return the text portion of the entire caption as a single string is needed. I have added it. This simplifies the test named above, and some code in Writer/Html. The latter is not part of unit testing because the version of JpGraph found in Composer is too antiquated. I verified the Html change manually by running samples/Chart/32_Chart_read_write_HTML.php using a recent version of JpGraph. It was as a result of this test that I uncovered issue #2203. I did not see anything about Charts in docs, so did not add a description of the new function there.
Phpstan is happy with the changes. We'll see how Scrutinizer feels when I push it.
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