Throw from addNamedEmbeddedFile when no ref is given

[MahathirMohammadShuvo]

Follow-up to #1771, as you asked — the throw on its own. lib/tree.js is
untouched, so the PDFTree.toString filter you were less inclined to merge is
not in here.

The bug

doc.addNamedEmbeddedFile('phantom.txt', undefined);

/EmbeddedFiles <<
  /Names [
    (phantom.txt) undefined
]
>>

undefined is not a PDF object, so the document does not parse. The caller asked
for an attachment under a name and got a broken file with no signal.

Why throw, of the three you listed

doc.file() already refuses the same thing:

doc.file(undefined, { name: 'x.txt' });    // Error: No src specified
doc.addNamedEmbeddedFile('x', undefined);  // silently accepted, corrupt output

addNamedEmbeddedFile is the lower-level route to the same place, so the two now
agree. Silent drop is the option I would argue against hardest — a valid PDF with
nothing attached and no error is harder to debug than a stack trace.

In addNamedEmbeddedFile rather than PDFTree.add, because add is shared with
/Dests, /JavaScript and ParentTree, which only ever receive values the
library builds itself.

Scope

!ref rather than === undefined, matching the if (!src) guard in file().
That also catches null, which is the more awkward of the two: it serialises to
the PDF null keyword, so the file parses and you get a silently empty
attachment rather than a loud broken one. Happy to narrow it if you would rather.

The guard sits before the lazy /EmbeddedFiles init, so a rejected call does not
leave an empty name tree behind in the catalog.

name is not validated. A bad key serialises to (undefined), which parses, and
the same applies to addNamedDestination and addNamedJavaScript — fixing it
here alone would make the three siblings inconsistent. Separate change.

The one caller inside the library, lib/mixins/attachments.js:102, passes a
filespec file() has just created, and file() throws before it can return
anything falsy — so the library's own path cannot reach the new guard.

Marked breaking, though anyone hitting this today is shipping a PDF that does not
parse.

Tests

2 added. throws when the ref is missing fails on master. The second registers a
filespec created with hidden: true and passes either way — a guard that the
normal hand-registration path still works, not evidence for the change.

Unit suite 396/396.