Fill all-NaN series with zeros in TimesFM 2.5 interpolation

[gyanu2507]

Summary

TimesFM 2.5 linear_interpolation falls back with if non_nans_values: when np.interp has no anchors. On NumPy 2.x that is ValueError: The truth value of an empty array is ambiguous, so an all-NaN series crashes before a forecast. v1 uses len(...) > 0 and TimesFM3 uses .size > 0, both filling 0.0.

Test plan

• all-NaN input becomes [0.0, 0.0, 0.0]
• existing interpolation cases still pass

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