qdev: accept empty string properties

These were stored as NULL due to wrong cut-and-paste from set_pointer.

Reported-by: Gerhard Wiesinger <lists@wiesinger.com>
Tested-by: Gerhard Wiesinger <lists@wiesinger.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/qdev-properties.c

diff --git a/hw/qdev-properties.c b/hw/qdev-properties.c
index 0423af1..bff9152 100644
--- a/hw/qdev-properties.c
+++ b/hw/qdev-properties.c
@@ -421,10 +421,6 @@
         error_propagate(errp, local_err);
         return;
     }
-    if (!*str) {
-        g_free(str);
-        str = NULL;
-    }
     if (*ptr) {
         g_free(*ptr);
     }