Refactor AssertJ assertions into more idiomatic ones

This commit refactors some AssertJ assertions into more idiomatic and
readable ones. Using the dedicated assertion instead of a generic one
will produce more meaningful error messages. 

For instance, consider collection size:
```
// expected: 5 but was: 2
assertThat(collection.size()).equals(5);
// Expected size: 5 but was: 2 in: [1, 2]
assertThat(collection).hasSize(5);
```

Closes gh-30104

spring-oxm/src/test/java/org/springframework/oxm/jaxb/Jaxb2MarshallerTests.java

@@ -287,7 +287,7 @@ class Jaxb2MarshallerTests extends AbstractMarshallerTests<Jaxb2Marshaller> {
 		BinaryObject object = new BinaryObject(bytes, dataHandler);
 		StringWriter writer = new StringWriter();
 		marshaller.marshal(object, new StreamResult(writer), mimeContainer);
-		assertThat(writer.toString().length() > 0).as("No XML written").isTrue();
+		assertThat(writer.toString()).as("No XML written").isNotEmpty();
 		verify(mimeContainer, times(3)).addAttachment(isA(String.class), isA(DataHandler.class));
 	}
 

spring-oxm/src/test/java/org/springframework/oxm/jaxb/Jaxb2UnmarshallerTests.java

@@ -66,7 +66,7 @@ public class Jaxb2UnmarshallerTests extends AbstractUnmarshallerTests<Jaxb2Marsh
 	protected void testFlights(Object o) {
 		Flights flights = (Flights) o;
 		assertThat(flights).as("Flights is null").isNotNull();
-		assertThat(flights.getFlight().size()).as("Invalid amount of flight elements").isEqualTo(1);
+		assertThat(flights.getFlight()).as("Invalid amount of flight elements").hasSize(1);
 		testFlight(flights.getFlight().get(0));
 	}
 
@@ -106,7 +106,7 @@ public class Jaxb2UnmarshallerTests extends AbstractUnmarshallerTests<Jaxb2Marsh
 		assertThat(condition).as("Result is not a BinaryObject").isTrue();
 		BinaryObject object = (BinaryObject) result;
 		assertThat(object.getBytes()).as("bytes property not set").isNotNull();
-		assertThat(object.getBytes().length > 0).as("bytes property not set").isTrue();
+		assertThat(object.getBytes()).as("bytes property not set").isNotEmpty();
 		assertThat(object.getSwaDataHandler()).as("datahandler property not set").isNotNull();
 	}
 

spring-oxm/src/test/java/org/springframework/oxm/xstream/XStreamMarshallerTests.java

@@ -339,7 +339,7 @@ class XStreamMarshallerTests {
 		marshaller.marshal(flight, new StreamResult(writer));
 		assertThat(writer.toString()).as("Invalid result").isEqualTo("{\"flight\":{\"flightNumber\":42}}");
 		Object o = marshaller.unmarshal(new StreamSource(new StringReader(writer.toString())));
-		assertThat(o instanceof Flight).as("Unmarshalled object is not Flights").isTrue();
+		assertThat(o).as("Unmarshalled object is not Flights").isInstanceOf(Flight.class);
 		Flight unflight = (Flight) o;
 		assertThat(unflight).as("Flight is null").isNotNull();
 		assertThat(unflight.getFlightNumber()).as("Number is invalid").isEqualTo(42L);
@@ -377,7 +377,7 @@ class XStreamMarshallerTests {
 	private static void assertXpathExists(String xPathExpression, String inXMLString){
 		Source source = Input.fromString(inXMLString).build();
 		Iterable<Node> nodes = new JAXPXPathEngine().selectNodes(xPathExpression, source);
-		assertThat(nodes).as("Expecting to find matches for Xpath " + xPathExpression).hasSizeGreaterThan(0);
+		assertThat(nodes).as("Expecting to find matches for Xpath " + xPathExpression).isNotEmpty();
 	}
 
 	private static void assertXpathDoesNotExist(String xPathExpression, String inXMLString){