Allow zero input for EpContext node

[shiyi (shiyi9801)]

Description

Set min_arity of inputs to zero in EPContext node registration.

Motivation and Context

PR #28771 relaxed CompileModel validation to accept zero-input OrtModel graphs, so EpContext node should also accept zero-input.

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[shiyi (shiyi9801)]

Hi adrastogi Dwayne Robinson (@fdwr) Please take a look, thanks!
This PR fixes an issue:

Failed to call ort_compile_api->CompileModel(env->get(), compile_options.get()): [WebNN] ORT status error code: 10 error message: This is an invalid model. In Node, ("OpenVINOExecutionProvider_OpenVINOExecutionProvider_3884444976624952189_0_0", EPContext, "com.microsoft", -1) : () -> ("output-1_1": tensor(float),"output-0_0": tensor(float),) , Error Node(OpenVINOExecutionProvider_OpenVINOExecutionProvider_3884444976624952189_0_0) with schema(com.microsoft::EPContext:1) has input size 0 not in range [min=1, max=2147483647].

[Dwayne Robinson (fdwr)]

👍

[Dwayne Robinson (fdwr)]

/azp run Test Linux CUDA x64 Release,Test Linux TensorRT x64 Release,web_Debug / build_onnxruntime_web,web_Release / build_onnxruntime_web

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[Dwayne Robinson (fdwr)]

Comment mostly for the ORT folks to assess any implications of this

adrastogi Do you want someone else from ORT to look at this? Otherwise I'm content to merge (I don't see how relaxing this restriction causes any compat issues, since ORT is backwards compatible, with no guarantee of forward compatibility with newer opsets or constraints).

[adrastogi]

Dwayne Robinson (@fdwr) thanks! No, I think we're good here, thanks for evaluating that comment.