Prevent working with empty document

[JorjMcKie]

Prevent page iterations if document has zero pages.

[JorjMcKie]

Addresses #4018.

[julian-smith-artifex-com]

Not sure we should raise an exception in this case. Could we instead do:

if not self.page_count:
    return

So that for page in empty_document.pages(): ... doesn't raise?

[JorjMcKie]

Not sure we should raise an exception in this case. Could we instead do:

if not self.page_count:
    return

So that for page in empty_document.pages(): ... doesn't raise?

Sure, why not. It is a very improbable situation anyway: the code lives like this for 5 years or so, without ever this has been happening.