Ensure floating point rasters have nodata set to NaN

[robbibt]

During 2026 annual product runs, we noticed an issue where output DEA Intertidal floating point rasters (e.g. elevation were missing the NoData Value=nan COG metadata tag. This meant that downstream workflows like COG mosaicking could not pick up the correct metadata value, and instead assigned 0 values outside the raster:

gdalinfo on '/vsicurl/https://data.dea.ga.gov.au/derivative/ga_s2ls_intertidal_cyear_3/2-0-0/x090/y100/2022--P1Y/ga_s2ls_intertidal_cyear_3_x090y100_2022--P1Y_final_elevation.tif':

Band 1 Block=512x512 Type=Float32, ColorInterp=Gray 
NoData Value=nan 
Overviews: 1600x1600, 800x800, 400x400, 200x200, 100x100

gdalinfo on '/vsicurl/https://data.dev.dea.ga.gov.au/derivative/ga_s2ls_intertidal_cyear_3/2-5-0/x090/y103/2022--P1Y/ga_s2ls_intertidal_cyear_3_x090y103_2022--P1Y_final_elevation.tif':

Band 1 Block=512x512 Type=Float32, ColorInterp=Gray 
Overviews: 1600x1600, 800x800, 400x400, 200x200, 100x100

This PR temporarily fixes this by ensuring that any output float data has nodata attributes manually set on the xarray prior to the data being written to COG.

[github-actions]

For full integration test results, refer to the Tests directory README.