Allow ngr_spk_stac_calc() to fetch single asset without calculation

[NewGraphEnvironment]

Modify ngr_spk_stac_calc() to:

• Allow asset_b to be NULL when only fetching a single asset
• Allow calc parameter to be NULL to skip spectral index calculation
• When calc = NULL, just return the asset clipped to the AOI (if provided)

This enables using the function for simple asset retrieval and clipping without requiring two bands or a calculation.