Fix "Trying to access array offset on value of type null" (#228)

Added a check that `$previous` is not null (i.e. previous token exists)

EmailValidator/Parser/Parser.php

@@ -170,7 +170,7 @@ abstract class Parser
     {
         $previous = $this->lexer->getPrevious();
 
-        if ($previous['type'] === EmailLexer::S_BACKSLASH
+        if ($previous && $previous['type'] === EmailLexer::S_BACKSLASH
             &&
             $this->lexer->token['type'] !== EmailLexer::GENERIC
         ) {