[WebNN EP] Make sure optional input is provided (#19686)
Some optional input is presented as empty string, we should not only
check if the input size is correct, but also check if the optional input
is not empty.
e.g. Pad node has empty optional input in sam-b-encoder.onnx model:
<img width="514" alt="image"
src="https://github.com/microsoft/onnxruntime/assets/3271201/cc3b06fe-46b9-4ee7-aca5-157bdf112856">
