syntax: Fix bugs found by static typing

Author: eemeli

fluent.syntax/fluent/syntax/parser.py

@@ -123,7 +123,7 @@ def get_entry_or_junk(self, ps: FluentParserStream) -> ast.EntryType:
             junk = ast.Junk(slice)
             if self.with_spans:
                 junk.add_span(entry_start_pos, next_entry_start)
-            annot = ast.Annotation(err.code, err.args, err.message)
+            annot = ast.Annotation(err.code, list(err.args) if err.args else None, err.message)
             annot.add_span(error_index, error_index)
             junk.add_annotation(annot)
             return junk
@@ -240,6 +240,9 @@ def get_attributes(self, ps: FluentParserStream) -> List[ast.Attribute]:
     @with_span
     def get_identifier(self, ps: FluentParserStream) -> ast.Identifier:
         name = ps.take_id_start()
+        if name is None:
+            raise ParseError('E0004', 'a-zA-Z')
+
         ch = ps.take_id_char()
         while ch:
             name += ch

fluent.syntax/fluent/syntax/serializer.py

@@ -82,6 +82,9 @@ def serialize_entry(self, entry: ast.EntryType, state: int = 0) -> str:
 
 
 def serialize_comment(comment: Union[ast.Comment, ast.GroupComment, ast.ResourceComment], prefix: str = "#") -> str:
+    if not comment.content:
+        return f'{prefix}\n'
+
     prefixed = "\n".join([
         prefix if len(line) == 0 else f"{prefix} {line}"
         for line in comment.content.split("\n")
@@ -91,7 +94,7 @@ def serialize_comment(comment: Union[ast.Comment, ast.GroupComment, ast.Resource
 
 
 def serialize_junk(junk: ast.Junk) -> str:
-    return junk.content
+    return junk.content or ''
 
 
 def serialize_message(message: ast.Message) -> str:
@@ -165,6 +168,7 @@ def serialize_placeable(placeable: ast.Placeable) -> str:
         return "{{ {}}}".format(serialize_expression(expr))
     if isinstance(expr, ast.Expression):
         return "{{ {} }}".format(serialize_expression(expr))
+    raise Exception('Unknown expression type: {}'.format(type(expr)))
 
 
 def serialize_expression(expression: Union[ast.Expression, ast.Placeable]) -> str: