Fix steep check offense

```
❯ bundle exec rake steep
bundle exec steep check
# Type checking files:

.............................................................................................................................F.........................

lib/lrama/grammar/symbols/resolver.rb:201:33: [error] Cannot pass a value of type `(::Integer | nil)` as an argument of type `::Integer`
│   (::Integer | nil) <: ::Integer
│     nil <: ::Integer
│
│ Diagnostic ID: Ruby::ArgumentTypeMismatch
│
└                   sym.token_id = Integer($1, 8)
                                   ~~~~~~~~~~~~~~

lib/lrama/grammar/symbols/resolver.rb:203:36: [error] Type `(::String | nil)` does not have method `bytes`
│ Diagnostic ID: Ruby::NoMethod
│
└                   sym.token_id = $1.bytes.first
                                      ~~~~~

Detected 2 problems from 1 file
```

Author: ydah

lib/lrama/grammar/symbols/resolver.rb

@@ -198,9 +198,17 @@ def fill_terms_number
                 when "\\\\"
                   sym.token_id = 92
                 when /\A\\(\d+)\z/
-                  sym.token_id = Integer($1, 8)
+                  unless (id = Integer($1, 8)).nil?
+                    sym.token_id = id
+                  else
+                    raise "Unknown Char s_value #{sym}"
+                  end
                 when /\A(.)\z/
-                  sym.token_id = $1.bytes.first
+                  unless (id = $1&.bytes&.first).nil?
+                    sym.token_id = id
+                  else
+                    raise "Unknown Char s_value #{sym}"
+                  end
                 else
                   raise "Unknown Char s_value #{sym}"
                 end