correct empty default parameter ins libdoc signature

Author: d-biehl

robotcode/language_server/robotframework/diagnostics/library_doc.py

@@ -433,11 +433,12 @@ def _get_signature(self, header_level: int, add_type: bool = True) -> str:
             result += "\n| | | | |"
             result += "\n|:--- | --:|:--|:---|"
             bs = "\\"
+
             for a in self.args:
                 result += (
                     f"\n| `{str(a.name)}`"
                     f"| {'=' if a.default_value is not None else ''}"
-                    f"| {f'`{str(a.default_value)}`' if a.default_value is not None else ''}"
+                    f"| {f'`{str(a.default_value)}`' if a.default_value else ''}"
                     f"| {' or '.join(f'`{bs}<{s}>`' for s in a.types) if a.types is not None else ''} |"
                 )