Improve WebGPU MatMulNBits to support zero pointer for 2bits #9740

Job	Run time
precheck	1m 41s
wasm_Release / build-wasm	50m 41s
wasm_Debug / build-wasm	53m 56s
wasm_Release_static_library / build-wasm	7m 32s
web_Release / build_onnxruntime_web	23m 27s
web_Debug / build_onnxruntime_web	18m 34s
	2h 35m 51s

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