Only set TARGET_PLATFORM version in lunch when explicitly requested

Setting TARGET_PLATFORM_VERSION to DEFAULT_PLATFORM_VERSION during
"lunch sailfish-userdebug" causes unnecessary pain when
DEFAULT_PLATFORM_VERSION becomes invalid after branching.  Only
set TARGET_PLATFORM_VERSION if it was explicitly requested with
lunch sailfish-userdebug-OPR1.

Test: build/make/tests/envsetup_tests.sh
Bug: 34972208
Bug: 37208937
Change-Id: I40ba4617e73803c5bec1a8d317382f70fb3ec3a2
diff --git a/tests/envsetup_tests.sh b/tests/envsetup_tests.sh
index 4aae255..abdcd56 100755
--- a/tests/envsetup_tests.sh
+++ b/tests/envsetup_tests.sh
@@ -19,8 +19,9 @@
 valid_version=PPR1
 
 # lunch tests
-check_lunch "aosp_arm64"                                "aosp_arm64" "eng"       "$default_version"
-check_lunch "aosp_arm64-userdebug"                      "aosp_arm64" "userdebug" "$default_version"
+check_lunch "aosp_arm64"                                "aosp_arm64" "eng"       ""
+check_lunch "aosp_arm64-userdebug"                      "aosp_arm64" "userdebug" ""
+check_lunch "aosp_arm64-userdebug-$default_version"     "aosp_arm64" "userdebug" "$default_version"
 check_lunch "aosp_arm64-userdebug-$valid_version"       "aosp_arm64" "userdebug" "$valid_version"
 check_lunch "abc"                                       "" "" ""
 check_lunch "aosp_arm64-abc"                            "" "" ""