Fix some spurious error messages in build_and_install.sh.

Also, update the Xcode versions for CI builds.

.travis.yml

@@ -1,11 +1,14 @@
 language: objective-c
 matrix:
   include:
+    - os: osx
+      osx_image: xcode12
+      sudo: required
+      env: DEPLOY=true
     - os: osx
       osx_image: xcode11.3
       xcode_sdk: macosx10.14
       sudo: required
-      env: DEPLOY=true
     - os: osx
       osx_image: xcode10.1
       xcode_sdk: macosx10.14
@@ -18,10 +21,6 @@ matrix:
       osx_image: xcode9.4
       xcode_sdk: macosx10.13
       sudo: required
-    - os: osx
-      osx_image: xcode9.2
-      xcode_sdk: macosx10.13
-      sudo: required
       # Fails to compile in 8.3, but it isn't clear from the logs why it fails.
       # - os: osx
       #   osx_image: xcode8.3

build_and_install.sh

@@ -663,7 +663,7 @@ else
 fi
 
 enableUBSanArg() {
-    if [[ -n "$ENABLE_UBSAN" ]]; then
+    if [[ "${ENABLE_UBSAN+}" != "" ]]; then
         echo "-enableUndefinedBehaviorSanitizer"
         echo "$ENABLE_UBSAN"
     fi