KM VTS test fix - handled "null" value for imei.

Added a check to make sure IMEI is not "null".

Bug: 281676499
Test: atest VtsAidlKeyMintTargetTest
Change-Id: Ia1569a30412d633eee4d4de8cd00dea077d1c23d
diff --git a/security/keymint/aidl/vts/functional/AttestKeyTest.cpp b/security/keymint/aidl/vts/functional/AttestKeyTest.cpp
index e759123..92f7ac9 100644
--- a/security/keymint/aidl/vts/functional/AttestKeyTest.cpp
+++ b/security/keymint/aidl/vts/functional/AttestKeyTest.cpp
@@ -80,7 +80,13 @@
         return "";
     }
 
-    return ::android::base::Trim(out[0]);
+    string imei = ::android::base::Trim(out[0]);
+    if (imei.compare("null") == 0) {
+        LOG(ERROR) << "Error in getting IMEI from Telephony service: value is null. Cmd: " << cmd;
+        return "";
+    }
+
+    return imei;
 }
 
 }  // namespace
@@ -1059,7 +1065,7 @@
 
     // Skip the test if there is no second IMEI exists.
     string second_imei = get_imei(1);
-    if (second_imei.empty() || second_imei.compare("null") == 0) {
+    if (second_imei.empty()) {
         GTEST_SKIP() << "Test not applicable as there is no second IMEI";
     }
 
@@ -1137,13 +1143,13 @@
 
     // Skip the test if there is no first IMEI exists.
     string imei = get_imei(0);
-    if (imei.empty() || imei.compare("null") == 0) {
+    if (imei.empty()) {
         GTEST_SKIP() << "Test not applicable as there is no first IMEI";
     }
 
     // Skip the test if there is no second IMEI exists.
     string second_imei = get_imei(1);
-    if (second_imei.empty() || second_imei.compare("null") == 0) {
+    if (second_imei.empty()) {
         GTEST_SKIP() << "Test not applicable as there is no second IMEI";
     }