Only decrypt if keys are present

Summary:
Travis doesn't set encrypted variables in pull requests (for very good reasons),
but openssl fails rather loudly if it doesn't get them. Our artifact publishing
script happily skips PRs anyway, so we don't need these variables in that case.

Reviewed By: emilsjolander

Differential Revision:
D5137092

Tags: accept2ship

fbshipit-source-id: b14d61feba0bb98a1705b2f8232f52ba59a2c734

.travis.yml

@@ -22,7 +22,11 @@ env:
   - TARGET=android
 
 before_install:
-  - openssl aes-256-cbc -K $encrypted_d27e803291ff_key -iv $encrypted_d27e803291ff_iv -in scripts/setup-keys.enc -d >> gradle.properties
+  - |
+    if [[ -n "$encrypted_d27e803291ff_iv" ]]; then
+      openssl aes-256-cbc -K $encrypted_d27e803291ff_key -iv $encrypted_d27e803291ff_iv -in scripts/setup-keys.enc -d >> gradle.properties;
+    fi
+
   - brew update > /dev/null
   - brew install --HEAD facebook/fb/buck