SHACL: bug in execution of SPARQL validation for a specified focusNode via validation API

Description:

While running validation focused on single explicit focus node.  It seems that due to the missing return statement validation will never be effective at all.

Reproduction:

While running validation focused on single resource node. E.g. by calling:

```

new ShaclPlainValidator().validate(shapes, data, node);

```

Example data:

```

<someResource>
     a <someClass> ;
    <someunknownProperty> <someUnimportantValue> ;
.

```

Example shape:

```

<propertyShape
    a              sh:NodeShape ;
    sh:severity    sh:Warning ;
    sh:target [
                  a sh:SPARQLTarget ;   
                  sh:select """
                        SELECT DISTINCT ?node
                        WHERE

{                             ?node a ?class .                         }

                        """ ;
              ] ;   
    sh:sparql      [ sh:message "Resource {$this} has unknown property {$path}" ;
                     sh:select """
                            SELECT DISTINCT $this ?path
                            WHERE {
                                $this ?path ?value .
                                $this a ?class .
                                FILTER NOT EXISTS

{                                      ?path <http://www.w3.org/2000/01/rdf-schema#domain> ?class                                 }

                            }
                        """ ] ;
.

```