validate() fails to report errors for invalid SPDX expressions #114

Problem

licensing.validate() method does not report errors for syntactically invalid SPDX license expressions that end with a trailing operator (e.g., AND or OR). According to the [SPDX License Expression Specification](https://spdx.github.io/spdx-spec/v3.0.1/annexes/spdx-license-expressions/), such expressions are invalid.

Steps to Reproduce

1. from license_expression import get_spdx_licensing
   licensing = get_spdx_licensing()
   expression = "GPL-3.0-or-later AND GPL-2.0-or-later AND GPL-2.0-only AND"
   result = licensing.validate(expression)
   print(result)

Expected Behavior

The licensing.validate() method should report an error because the expression GPL-3.0-or-later AND GPL-2.0-or-later AND GPL-2.0-only AND is syntactically invalid due to the trailing AND.

Actual Behavior

The method returns ExpressionInfo with errors=[] and invalid_symbols=[], indicating no issues, even though the expression is malformed:

ExpressionInfo(
    original_expression='GPL-3.0-or-later AND GPL-2.0-or-later AND GPL-2.0-only AND',
    normalized_expression='GPL-3.0-or-later AND GPL-2.0-or-later AND GPL-2.0-only',
    errors=[],
    invalid_symbols=[]
)