[Spectral Index Creator app] invalid output if input raster is a VRT

[Andreas Rabe]

Reported by Henrike.

• Running algo with Sentinel 2 vrt-file gives error:
  name 'nan' is not defined,, name 'nan' is not defined,, name 'nan' is not defined, name 'nan' is not defined

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[Andreas Rabe]

https://github.com/EnMAP-Box/enmap-box/commit/c8d2760da46426c73a364af12ba43d61adfc444f