Mask operator accepts an empty masking_char and deletes the masked characters #2486

Describe the bug
The mask operator accepts an empty masking_char. Validation only rejects values longer than one character, so "" passes. operate then builds "" * chars_to_mask + text[chars_to_mask:], so the masked characters are deleted, not masked. The output is shorter than the input and keeps the unmasked part of the value.

Code:

To Reproduce
On main (aa77b9f):

from presidio_anonymizer import AnonymizerEngine
from presidio_anonymizer.entities import OperatorConfig, RecognizerResult

result = AnonymizerEngine().anonymize(
    "My card 4111111111111111 ok",
    [RecognizerResult("CREDIT_CARD", 8, 24, 1.0)],
    {"CREDIT_CARD": OperatorConfig(
        "mask", {"masking_char": "", "chars_to_mask": 12, "from_end": False}
    )},
)
print(repr(result.text))  # 'My card 1111 ok'

Expected behavior
InvalidParamError, like for a masking_char of two characters ("masking_char must be a character").

Actual behavior
No error. The first 12 digits are removed and the result is 'My card 1111 ok'.

Additional context
The check is len(masking_char) > 1; len(masking_char) != 1 would cover the empty string too. The same request through the anonymizer REST API ("masking_char": "") behaves the same way.

I have a fix with a regression test and will open a PR.