route directive: ignore empty header values

policy/v1beta1/cfg.proto

@@ -131,12 +131,17 @@ message Rule {
   // that may reference action outputs by name. For example, if an action `x` produces an output
   // with a field `f`, then the header value expressions may use attribute `x.output.f` to reference
   // the field value:
+  //
   // ```yaml
   // request_header_operations:
   // - name: x-istio-header
   //   values:
   //   - x.output.f
   // ```
+  //
+  // If the header value expression evaluates to an empty string, and the operation is to either replace
+  // or append a header, then the operation is not applied. This permits conditional behavior on behalf of the
+  // adapter to optionally modify the headers.
   message HeaderOperationTemplate {
 
     // Required. Header name literal value.