Fix bug when EMC doesn't have a balancing BMC/BDC (#707)

* Fix bug when EMC doesn't have a balancing BMC/BDC

* Change error message in OperatorParser

Author: LonelyMidoriya

validation-model/src/main/java/org/verapdf/gf/model/factory/operators/OperatorParser.java

@@ -191,7 +191,7 @@ void parseOperator(List<org.verapdf.model.operator.Operator> processedOperators,
 				if (!this.markedContentStack.empty()) {
 					this.markedContentStack.pop();
 				} else {
-					LOGGER.log(Level.WARNING, getErrorMessage("Operator (EMC) not inside marked content"));
+					LOGGER.log(Level.WARNING, getErrorMessage("EMC operator does not have a balancing BMC/BDC operator"));
 				}
 				break;
 			case Operators.MP:

wcag-validation/src/main/java/org/verapdf/gf/model/factory/chunks/ChunkParser.java

@@ -106,7 +106,11 @@ public void parseChunk(Operator rawOperator, List<COSBase> arguments) {
 				break;
 			case Operators.EMC:
 				processLineArts();
-				markedContentStack.pop();
+                if (!markedContentStack.isEmpty()) {
+                    markedContentStack.pop();
+                } else {
+                    LOGGER.log(Level.WARNING, "EMC operator does not have a balancing BMC/BDC operator");
+                }
 				break;
 			case Operators.G_FILL: {
 				if (this.graphicsState.isProcessColorOperators()) {