Do not crash when adding an image inside a table cell

[someth2say]

A minimal reproducer would be:

|===
a|
image::image.svg[]
|===

If trying to generate the ePub leads to a crash:

$ asciidoctor-epub3 book.adoc
undefined method `attr?' for nil:NilClass

The reason is that the method get_enclosing_chapter returns nil when inside a table cell document.