Allow no input in str constructor

Author: msimacek

graalpython/com.oracle.graal.python.test/src/tests/unittest_tags/test_unicode.txt

@@ -18,6 +18,7 @@
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_comparison
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_concatenation
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_constructor
+*graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_constructor_defaults
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_constructor_keyword_args
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_contains
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_count

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/BuiltinConstructors.java

@@ -1797,9 +1797,9 @@ public final Object executeWith(VirtualFrame frame, Object arg) {
 
         public abstract Object executeWith(VirtualFrame frame, Object strClass, Object arg, Object encoding, Object errors);
 
-        @Specialization(guards = {"!isNativeClass(strClass)", "isNoValue(arg)", "isNoValue(encoding)", "isNoValue(errors)"})
+        @Specialization(guards = {"!isNativeClass(strClass)", "isNoValue(arg)"})
         @SuppressWarnings("unused")
-        Object strNoArgs(Object strClass, PNone arg, PNone encoding, PNone errors) {
+        Object strNoArgs(Object strClass, PNone arg, Object encoding, Object errors) {
             return asPString(strClass, "");
         }