python-info: avoid setting python_info[version] if python is not on the PATH

modules/python/functions/python-info

@@ -32,10 +32,12 @@ if [[ -n "$CONDA_DEFAULT_ENV" ]]; then
   python_info[virtualenv]="$virtualenv_formatted"
 fi
 
+if (( $+commands[python] )); then
   zstyle -s ':prezto:module:python:info:version' format 'version_format'
   if [[ -n "$version_format" ]]; then
     zformat -f version_formatted "$version_format" "v:${$(python3 --version)#Python }"
     python_info[version]="$version_formatted"
   fi
+fi
 
 # }