handle missing secrets

Missing secrets will give a blank string instead of exploding.

Also, an example of a custom property

Shared/Secrets.swift.gyb

@@ -18,11 +18,11 @@ def snake_to_camel(snake_str):
 salt = [ord(byte) for byte in os.urandom(64)]
 }%
 enum Secrets {
-
   % for secret in secrets:
+
   static var ${snake_to_camel(secret)}: String {
      let encoded: [UInt8] = [
-      % for chunk in chunks(encode(os.environ.get(secret), salt), 8):
+      % for chunk in chunks(encode(os.environ.get(secret) or "", salt), 8):
           ${"".join(["0x%02x, " % byte for byte in chunk])}
       % end
       ]
@@ -30,7 +30,10 @@ enum Secrets {
       return decode(encoded, salt: salt)
   }
   % end
-  
+
+  %{
+  # custom example: static let myVariable = "${os.environ.get('MY_CUSTOM_VARIABLE')}"
+  }%
 }
 
 private extension Secrets {