pro2cmake: skip tuple declaration on condition str

In some cases after cleaning and simplifying the conditions
you can end up with the following line:
    ((()))
Python interprets that as an empty tuple:
    >>> a = ((()))
    >>> a
    ()
and then the simplify_condition (sympy) method fails to continue
with an error:
      File ".../util/cmake/pro2cmake.py", line 1993, in simplify_condition
        condition_expr = simplify_logic(condition)
      File ".../sympy/logic/boolalg.py", line 2292, in simplify_logic
        expr = expr.xreplace(dict(zip(variables, s)))
    AttributeError: 'tuple' object has no attribute 'xreplace'

You can see this behavior with the file:
    qtmultimedia/tests/auto/unit/multimediaqml.pro

Change-Id: Ied4285b67f1e567660be49f6610311199bc48a22
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Qt CMake Build Bot

util/cmake/pro2cmake.py

@@ -1598,6 +1598,8 @@ def handle_subdir(
                 if condition_str and subtraction_str:
                     condition_str += " AND "
                 condition_str += subtraction_str
+                if not condition_str.rstrip("()").strip():
+                    continue
                 condition_simplified = simplify_condition(condition_str)
                 condition_key = condition_simplified