WifiService: allow EAP variables to be unset using empty strings.
diff --git a/services/java/com/android/server/WifiService.java b/services/java/com/android/server/WifiService.java
index d5de1f0..7951fb7 100644
--- a/services/java/com/android/server/WifiService.java
+++ b/services/java/com/android/server/WifiService.java
@@ -923,7 +923,7 @@
                 String value = field.value();
                 if (value != null) {
                     if (field != config.eap) {
-                        value = convertToQuotedString(value);
+                        value = (value.length() == 0) ? "NULL" : convertToQuotedString(value);
                     }
                     if (!WifiNative.setNetworkVariableCommand(
                                 netId,