[DotNet-Automation] Package generation failure would set 'installInstructions' to null which cause automation failure

[raych1]

Description
For a spec PR containing the spec changes from multiple service folders, we need to generate them one by one and any failure of one package generation should not block other package generation. Currently, a package generation failure will set installInstructions to 'null' in the generateOutput.json, this will end the automation script execution.

Expected
installInstructions should not be set or set correctly, otherwise it would fail the schema validation.

Screenshot

Sample pipeline run
https://dev.azure.com/azure-sdk/internal/_build/results?buildId=3913620&view=logs&j=03afb3bb-7296-55ad-aa07-ceee610c73b2&t=0f320c29-1c90-5304-d2d3-c58a6bb76437

//CC: @chunyu3