Fix known errors so far

Parse error  class checks for correct number of arguments to its emitter
and does seem to have correctly complained about the empty tag errors

Duplicate attributes should not be checked if the token is an end tag

Finally, used null coalescing to silence an undefined variable

lib/Tokenizer.php

@@ -443,7 +443,7 @@ class Tokenizer {
 
                     // Making errors more expressive.
                     if ($char !== '') {
-                        ParseError::trigger(ParseError::TAG_NAME_EXPECTED, $char);
+                        ParseError::trigger(ParseError::TAG_NAME_EXPECTED);
                     } else {
                         ParseError::trigger(ParseError::UNEXPECTED_EOF);
                     }
@@ -483,7 +483,7 @@ class Tokenizer {
                 # ">" (U+003E)
                 elseif ($char === '>') {
                     # Parse error. Switch to the data state.
-                    ParseError::trigger(ParseError::TAG_NAME_EXPECTED, $char);
+                    ParseError::trigger(ParseError::TAG_NAME_EXPECTED);
                     $this->state = self::DATA_STATE;
                 }
                 # EOF
@@ -1511,7 +1511,7 @@ class Tokenizer {
                     }
 
                     // Need to add the current attribute to the token, if necessary.
-                    if ($attribute) {
+                    if ($attribute ?? null) {
                         $token->attributes[] = $attribute;
                         $attribute = null;
                     }
@@ -1546,7 +1546,7 @@ class Tokenizer {
                 }
                 # "=" (U+003D)
                 elseif ($char === '=') {
-                    if ($token->hasAttribute($attribute->name)) {
+                    if ($token instanceof StartTagToken && $token->hasAttribute($attribute->name)) {
                         ParseError::trigger(ParseError::ATTRIBUTE_EXISTS, $attribute->name);
                     }