nginx: Empty the robots.txt file when there are no rules (#5013)
nginx: Empty the robots.txt file when there are no rules (#5013)

Merging this as it is pretty well understood. 
Thank you for your contribution!
arbrandes/edx-configuration,appsembler/configuration,stvstnfrd/configuration,EDUlib/configuration,arbrandes/edx-configuration,edx/configuration,stvstnfrd/configuration,stvstnfrd/configuration,stvstnfrd/configuration,mitodl/configuration,EDUlib/configuration,arbrandes/edx-configuration,mitodl/configuration,arbrandes/edx-configuration,appsembler/configuration,appsembler/configuration,edx/configuration,arbrandes/edx-configuration,mitodl/configuration,mitodl/configuration,EDUlib/configuration,EDUlib/configuration,EDUlib/configuration,appsembler/configuration,stvstnfrd/configuration,edx/configuration,edx/configuration