Extra nil safety in PageState

[yob]

Some of the text state operators mutate instance variables like @text_matrix and @text_line_matrix, and assume the instance variables exist.

In valid PDFs the text state operators that do so are within BT text block, so it's fine for us to initialize the instance variables there.

However, invalid PDFs (like those genrated by the fuzzer) might have text state operators without a preceding BT operator. For safety, initialize these vars in the constructor.

[yob]

Alternatively, we could make text state operators a no-op when outside a BT block. 🤔

[yob]

at the very least, I might add some PageState unit specs for calling the text state operators out of order, just to confirm they don't raise