Make it so that you can define expression-only rules (#380)

Same as bufbuild/protovalidate-go#288

Signed-off-by: Sri Krishna <[email protected]>

Author: srikrsna-buf

src/main/java/build/buf/protovalidate/CompiledProgram.java

@@ -105,8 +105,12 @@ final class CompiledProgram {
       if (Boolean.TRUE.equals(value)) {
         return null;
       }
+      String message = this.source.message;
+      if (message.isEmpty()) {
+        message = String.format("\"%s\" returned false", this.source.expression);
+      }
       RuleViolation.Builder builder =
-          RuleViolation.newBuilder().setRuleId(this.source.id).setMessage(this.source.message);
+          RuleViolation.newBuilder().setRuleId(this.source.id).setMessage(message);
       if (rulePath != null) {
         builder.addAllRulePathElements(rulePath.getElementsList());
       }