Improve null-safety of JsonContentAssert

See gh-46926

Author: mhalbritter

core/spring-boot-test/src/main/java/org/springframework/boot/test/json/JsonContentAssert.java

@@ -1000,9 +1000,11 @@ private JSONCompareResult compare(@Nullable CharSequence expectedJson, JSONCompa
 		if (this.actual == null) {
 			return compareForNull(expectedJson);
 		}
+		if (expectedJson == null) {
+			return fail("Expected JSON but got null");
+		}
 		try {
-			return JSONCompare.compareJSON((expectedJson != null) ? expectedJson.toString() : null,
-					this.actual.toString(), compareMode);
+			return JSONCompare.compareJSON(expectedJson.toString(), this.actual.toString(), compareMode);
 		}
 		catch (Exception ex) {
 			if (ex instanceof RuntimeException runtimeException) {
@@ -1016,9 +1018,11 @@ private JSONCompareResult compare(@Nullable CharSequence expectedJson, JSONCompa
 		if (this.actual == null) {
 			return compareForNull(expectedJson);
 		}
+		if (expectedJson == null) {
+			return fail("Expected JSON but got null");
+		}
 		try {
-			return JSONCompare.compareJSON((expectedJson != null) ? expectedJson.toString() : null,
-					this.actual.toString(), comparator);
+			return JSONCompare.compareJSON(expectedJson.toString(), this.actual.toString(), comparator);
 		}
 		catch (Exception ex) {
 			if (ex instanceof RuntimeException runtimeException) {
@@ -1037,6 +1041,12 @@ private JSONCompareResult compareForNull(@Nullable CharSequence expectedJson) {
 		return result;
 	}
 
+	private JSONCompareResult fail(String message) {
+		JSONCompareResult result = new JSONCompareResult();
+		result.fail(message);
+		return result;
+	}
+
 	private JsonContentAssert assertNotFailed(JSONCompareResult result) {
 		if (result.failed()) {
 			failWithMessage("JSON Comparison failure: %s", result.getMessage());