How to use java 8 merge function for n number of hashmaps

I need to merge n hashmaps ideally in a loop like shown below using java 8 merge or something:

Map 1 : {Name:XXX,Phn:123,Work:""}

Map 2 : {Name:XXX,Phn:456,Work: xyz}

Map 3 : {Name:XXX,Phn:789,Work:""}

I would like to get an output like below :

{
   Name:XXX,        // if all values for a key are same take one
   Phn:123/456/789  // merge all non null values for same key
   Work:xyz         // make sure non-null values are never replaced
}

When I try to use putall like this

public Map<String,String> mergeOriginalDataMaps(List<Integer> indexList, List<Map<String,String>> originalData) {   
    Map<String,String> tmpMap = new HashMap<String,String> ();
    for (int index : indexList ) {
        tmpMap.putAll(originalData.get(index));
    }
    return tmpMap;
}

If the value of duplicate key is "", the previous value is replaced with the new one. I need to concatenate the values rather than replace them.