wpa_gui-qt4: Unset string variables instead of setting them to ""

This allows identity (etc.) variables to be removed from configuration
which is different from setting them to an empty string. For example,
EAP-SIM and EAP-AKA can now be configured to use identity string generation
from SIM/USIM by clearing the identity string in wpa_gui.

wpa_supplicant/wpa_gui-qt4/networkconfig.cpp

@@ -283,6 +283,8 @@ void NetworkConfig::addNetwork()
 		setNetworkParam(id, "eap", eap, false);
 		if (strcmp(eap, "SIM") == 0 || strcmp(eap, "AKA") == 0)
 			setNetworkParam(id, "pcsc", "", true);
+		else
+			setNetworkParam(id, "pcsc", "NULL", false);
 	}
 	if (phase2Select->isEnabled()) {
 		QString eap = eapSelect->currentText();
@@ -313,22 +315,32 @@ void NetworkConfig::addNetwork()
 					 "auth=GTC MSCHAPV2");
 			}
 		}
-		setNetworkParam(id, "phase2", phase2, true);
-	}
-	if (identityEdit->isEnabled())
+		if (phase2[0])
+			setNetworkParam(id, "phase2", phase2, true);
+		else
+			setNetworkParam(id, "phase2", "NULL", false);
+	} else
+		setNetworkParam(id, "phase2", "NULL", false);
+	if (identityEdit->isEnabled() && identityEdit->text().length() > 0)
 		setNetworkParam(id, "identity",
 				identityEdit->text().toAscii().constData(),
 				true);
-	if (passwordEdit->isEnabled() &&
+	else
+		setNetworkParam(id, "identity", "NULL", false);
+	if (passwordEdit->isEnabled() && passwordEdit->text().length() > 0 &&
 	    strcmp(passwordEdit->text().toAscii().constData(),
 		   WPA_GUI_KEY_DATA) != 0)
 		setNetworkParam(id, "password",
 				passwordEdit->text().toAscii().constData(),
 				true);
-	if (cacertEdit->isEnabled())
+	else if (passwordEdit->text().length() == 0)
+		setNetworkParam(id, "password", "NULL", false);
+	if (cacertEdit->isEnabled() && cacertEdit->text().length() > 0)
 		setNetworkParam(id, "ca_cert",
 				cacertEdit->text().toAscii().constData(),
 				true);
+	else
+		setNetworkParam(id, "ca_cert", "NULL", false);
 	writeWepKey(id, wep0Edit, 0);
 	writeWepKey(id, wep1Edit, 1);
 	writeWepKey(id, wep2Edit, 2);
@@ -343,10 +355,12 @@ void NetworkConfig::addNetwork()
 	else if (wep3Radio->isEnabled() && wep3Radio->isChecked())
 		setNetworkParam(id, "wep_tx_keyidx", "3", false);
 
-	if (idstrEdit->isEnabled())
+	if (idstrEdit->isEnabled() && idstrEdit->text().length() > 0)
 		setNetworkParam(id, "id_str",
 				idstrEdit->text().toAscii().constData(),
 				true);
+	else
+		setNetworkParam(id, "id_str", "NULL", false);
 
 	if (prioritySpinBox->isEnabled()) {
 		QString prio;