ROB: AppearanceStream: Remove -1 key from font.character_map

This patch adds back some code that got removed earlier and,
at that time, did not see any test coverage. With new code
that enables adding fonts, I've finally understood that, in
some cases, a -1 key will be added to font.character_map.
This will cause an encoding failure when generating
font_glyph_byte_map.

Author: PJBrs

pypdf/generic/_appearance_stream.py

@@ -393,11 +393,14 @@ def __init__(
                 __name__
             )
 
-        font_glyph_byte_map: dict[str, bytes]
+        font_glyph_byte_map: dict[str, bytes] = {}
         if isinstance(font.encoding, str):
-            font_glyph_byte_map = {
-                v: k.encode(font.encoding) for k, v in font.character_map.items()
-            }
+            try:  # Remove -1 key that does not map a character
+                del font.character_map[-1]
+            except KeyError:
+                pass
+            for k, v in font.character_map.items():
+                font_glyph_byte_map[v] = k.encode(font.encoding)
         else:
             font_glyph_byte_map = {v: bytes((k,)) for k, v in font.encoding.items()}
             font_encoding_rev = {v: bytes((k,)) for k, v in font.encoding.items()}