Java 8 Filtering with condition and collecting custom Map

Just check, whether you need to apply the filter or not and then use the filter method or don't use it:

protected List<Map<String, Object>> populate(List<SomeObject> someObjects, String string) {
    Stream<SomeObject> stream = someObjects.stream();
    if (string != null) {
         stream = stream.filter(s -> string.equals(s.getName()));
    }
    return stream.map(someObject -> {
        Map<String, Object> map = new LinkedHashMap<>();
        map.put("someCustomField1", someObject.Field1());
        map.put("someCustomField2", someObject.Field2());
        map.put("someCustomField3", someObject.Field3());
        return map;
    }).collect(Collectors.toList());
}

Try with this:

protected List<Map<String, Object>> populate(List<SomeObject> someObjects, String string) {
    return someObjects.stream()
            .filter(someObject -> string == null || string.equals(someObject.getName()))
            .map(someObject -> 
              new HashMap<String, Object>(){{
                    put("someCustomField1", someObject.Field1());
                    put("someCustomField2", someObject.Field2());
                    put("someCustomField3", someObject.Field3());
              }})
            .collect(Collectors.toList()) ;
}