clarified that missing attributes should not result in errors

Signed-off-by: Calum Murray <cmurray@redhat.com>

cesql/spec.md

@@ -206,6 +206,9 @@ by expected or unexpected errors. Nevertheless CESQL includes the concept of err
 case an error arises, the evaluator collects a list of errors, referred in this spec as _error list_, which is then
 returned together with the evaluated value of the CESQL expression.
 
+Addressing an attribute which is missing from the input event MUST not return an error due to the missing attribute. Instead, 
+an empty string MUST be assumed as its value, and execution MUST proceed as it would with any other string value.
+
 Whenever possible, some error checks SHOULD be done at compile time by the expression evaluator, in order to prevent
 runtime errors.