Fix crash on malformed SQL like "{ ="

[eeshsaxena]

The bug

from sql_metadata import Parser
Parser("{ =").columns
# AttributeError: 'NoneType' object has no attribute 'name'

Every public accessor crashes on the same three-character input: .tables, .columns, .query_type, .tables_aliases, .columns_dict. SELECT { = and x { = behave the same.

Why

DialectParser._parse_with_dialect runs sqlglot with ErrorLevel.WARN so it returns a best-effort partial AST instead of raising on the first problem. For { = sqlglot builds a bracket/map node whose key is None and then raises AttributeError while evaluating e.this.name on it. That is neither ParseError nor TokenError, so _try_dialects (which only catches those two) never sees it and the exception propagates all the way out to the accessors, even though the method is documented to raise InvalidQueryDefinition when no dialect produces a usable AST.

The fix

Catch any sqlglot-side failure in _parse_with_dialect and treat it as 'this dialect produced no result' (returns None), so a malformed query is reported through the normal InvalidQueryDefinition path rather than crashing. ParseError/TokenError are re-raised so the existing syntax-error handling is untouched.

After the fix { = no longer crashes: query_type/tables raise InvalidQueryDefinition and the best-effort accessors come back empty.

Tests

Added test/test_malformed_input.py covering the three inputs across the accessors. Full suite passes and dialect_parser.py stays at 100% coverage.