fix allowing empty {Ascii,Byte}Escape

Because of the leading pipe, |, these categories were permitted to be
empty:

    AsciiEscape ::=
      | $$\0$$
      | $$\"$$
      ..

Because of that, CharacterLiteral was also allowed to be empty:

    CharacterLiteral ::=
        $$'$$ CharacterContent $$'$$

    CharacterContent ::=
        AsciiEscape
      | CharacterLiteralCharacter
      | UnicodeEscape

Same for ByteEscape and ByteLiteral. These were the only two categories:

    $ rg -U '::=\n +\|' | wc -l
    0

Author: rbakbashev

src/lexical-elements.rst

@@ -574,7 +574,7 @@ Byte Literals
      | ByteEscape
 
    ByteEscape ::=
-     | $$\0$$
+       $$\0$$
      | $$\"$$
      | $$\'$$
      | $$\t$$
@@ -1102,7 +1102,7 @@ Character Literals
      | UnicodeEscape
 
    AsciiEscape ::=
-     | $$\0$$
+       $$\0$$
      | $$\"$$
      | $$\'$$
      | $$\t$$