allow nullptr string values (closes #138)

example/example14.cpp

@@ -29,4 +29,6 @@ void init_ex14(py::module &m) {
 
     m.def("return_void_ptr", []() { return (void *) 1234; });
     m.def("print_void_ptr", [](void *ptr) { std::cout << "Got void ptr : " << (uint64_t) ptr << std::endl; });
+    m.def("return_null_str", []() { return (char *) nullptr; });
+    m.def("print_null_str", [](char *ptr) { std::cout << "Got null str : " << (uint64_t) ptr << std::endl; });
 }

example/example14.py

@@ -4,6 +4,8 @@ import sys
 sys.path.append('.')
 
 from example import StringList, print_opaque_list
+from example import return_void_ptr, print_void_ptr
+from example import return_null_str, print_null_str
 
 l = StringList()
 l.push_back("Element 1")
@@ -13,6 +15,7 @@ print("Back element is %s" % l.back())
 l.pop_back()
 print_opaque_list(l)
 
-from example import return_void_ptr, print_void_ptr
-
 print_void_ptr(return_void_ptr())
+
+print(return_null_str())
+print_null_str(return_null_str())

example/example14.ref

@@ -5,3 +5,5 @@ Back element is Element 2
 Opaque list: 
   Element 1
 Got void ptr : 1234
+None
+Got null str : 0