ROB: Accept empty object as null objects (#1477)

Fixes #1474

PyPDF2/generic/_data_structures.py

@@ -847,6 +847,9 @@ def read_object(
         return BooleanObject.read_from_stream(stream)
     elif tok == b"(":
         return read_string_from_stream(stream, forced_encoding)
+    elif tok == b"e" and stream.read(6) == b"endobj":
+        stream.seek(-6, 1)
+        return NullObject()
     elif tok == b"n":
         return NullObject.read_from_stream(stream)
     elif tok == b"%":
@@ -869,7 +872,7 @@ def read_object(
         else:
             return NumberObject.read_from_stream(stream)
     else:
-        stream.read(-20)
+        stream.seek(-20, 1)
         raise PdfReadError(
             f"Invalid Elementary Object starting with {tok!r} @{stream.tell()}: {stream.read(80).__repr__()}"
         )

tests/test_generic.py

@@ -292,6 +292,20 @@ def test_read_object_comment_exception():
     assert exc.value.args[0] == "File ended unexpectedly."
 
 
+def test_read_object_empty():
+    stream = BytesIO(b"endobj")
+    pdf = None
+    assert isinstance(read_object(stream, pdf), NullObject)
+
+
+def test_read_object_invalid():
+    stream = BytesIO(b"hello")
+    pdf = None
+    with pytest.raises(PdfReadError) as exc:
+        read_object(stream, pdf)
+    assert "hello" in exc.value.args[0]
+
+
 def test_read_object_comment():
     stream = BytesIO(b"% foobar\n1 ")
     pdf = None