Fix out-of-bounds read in optimizer passes with zero-element initializers

[GopalakrishnanN]

Description

Multiple graph optimizer passes crashed with SIGSEGV when processing zero-element constant initializers (shape [0], 0 bytes of data). These are valid ONNX tensors that pass validation but caused out-of-bounds memory access when data<T>()[0] was dereferenced without checking size().

This is the same bug pattern fixed in relu_clip_fusion.cc by PR #26878, but the fix was never propagated to these passes.

Fixes

• div_mul_fusion.cc: Changed size() > 1 to size() != 1 to reject both zero-element and multi-element initializers
• layer_norm_fusion.cc: Added size() > 0 guard before epsilon access in both LayerNormFusion and SimplifiedLayerNormFusion passes, falling back to DEFAULT_LAYERNORM_EPSILON
• dropout_elimination.cc: Added size() == 0 early return before ratio data access
• double_qdq_pairs_remover.cc: Added size() == 0 checks for zero-point and scale initializers
• utils.cc: Added size() == 0 check in GetClipConstantMinMax before accessing Clip min/max initializer data

Tests

Added 4 regression tests verifying zero-element initializers don't crash:

• DivMulFusion_ZeroElementInitializer_NoCrash
• NoopElimination_ZeroElementInitializer_NoCrash
• LayerNormFusion_ZeroElementEpsilon_NoCrash
• SimplifiedLayerNormFusion_ZeroElementEpsilon_NoCrash

All 48 related optimizer tests pass (44 pre-existing + 4 new).

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