Support corrupt documents with *empty* Name-entries (issue 13610)

Apparently some really bad PDF software can create documents with *empty* `Name`-entries, which we thus need to somehow deal with.
While I don't know if this patch is necessarily the best solution, it should at least ensure that the *empty* `Name`-instance cannot accidentally match a proper `Name`-instance (and it doesn't require changes to a lot of existing code).[1]

---
[1] I briefly considered using a `Symbol` rather than an Object, but quickly decided against that since the former one [is not clonable](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm#supported_types) and `Name`-instances may be sent to the API.

src/core/parser.js

@@ -1113,6 +1113,9 @@ class Lexer {
     }
     if (strBuf.length > 127) {
       warn(`Name token is longer than allowed by the spec: ${strBuf.length}`);
+    } else if (strBuf.length === 0) {
+      warn("Name token is empty.");
+      return Name.empty;
     }
     return Name.get(strBuf.join(""));
   }

src/core/primitives.js

@@ -33,6 +33,12 @@ const Name = (function NameClosure() {
       return nameValue ? nameValue : (nameCache[name] = new Name(name));
     }
 
+    static get empty() {
+      // eslint-disable-next-line no-restricted-syntax
+      const emptyName = new Name({ empty: true });
+      return shadow(this, "empty", emptyName);
+    }
+
     static _clearCache() {
       nameCache = Object.create(null);
     }