Do not read metadata from non-existing djvu image

Do not read metadata from non-existing djvu image

Tests are printing:
*** [1-11711] Failed to open 'some-nonexistent-file': No such file or
directory.
*** (ByteStream.cpp:693)
*** 'DJVU::GUTF8String DJVU::ByteStream::Stdio::init(const DJVU::GURL&,
const char*)'
djvutxt: [1-11711] Failed to open 'some-nonexistent-file': No such file
or directory.
djvutxt: 'ByteStream.cpp:693'

That just spamming the logs, so adding a valid check before do anything.

Change-Id: Ia77377e632587ede76af4c722a1b8ac34fa50e1c