Allow obsproc_run_ver to be unset in config.base

Add ":-" to obsproc_run_ver variable setting for HOMEobsproc;
the run.spack.ver version file is not currently sourced during
forecast jobs and thus obsproc_run_ver is not set; it's not
needed for forecast jobs however so allow it to be unset

Refs NOAA-EMC#2291

parm/config/gfs/config.base

@@ -95,7 +95,7 @@ export MODE="@MODE@" # cycled/forecast-only
 # Build paths relative to $HOMEgfs
 export FIXgsi="${HOMEgfs}/fix/gsi"
 export HOMEpost="${HOMEgfs}"
-export HOMEobsproc="${BASE_GIT:-}/obsproc/v${obsproc_run_ver}"
+export HOMEobsproc="${BASE_GIT:-}/obsproc/v${obsproc_run_ver:-}"
 
 # CONVENIENT utility scripts and other environment parameters
 export NCP="/bin/cp -p"