check for unitialized AS variables

When a variable is uninitialized in the AS configuration, there is currently no way to detect it and this leads to unexpected results.

for example this code snippet :

[[ "%LPJG_CONFIG%" = *[!\ ]* ]] && lpjg=%LPJG_CONFIG% || lpjg=""

leads to the following result if LPJG_CONFIG is not defined:

lpjg=%LPJG_CONFIG%

whereas I would like lpjg="" as a result.

For the NFIXYR variable in auto-ecearth we use this workaround in test_model but it only works because we expect it to be a number, so it would not work in this case where we expect a string.

    if [[ -z $NFIXYR  ]]; then
        echo $error_NFIXYR && get_out=true
    else
        if !(( $NFIXYR >= 0 )); then  echo $error_NFIXYR; get_out=true; fi
    fi

Can I please have a fast answer @dmanubens @lbatista ? Thanks