Override containsProperty() in FallbackEnvironmentPropertySource

This commit overrides containsProperty() in
FallbackEnvironmentPropertySource for consistency with the
implementation of ConfigurableEnvironmentPropertySource.

See gh-34861

Author: sbrannen

spring-context/src/main/java/org/springframework/context/support/PropertySourcesPlaceholderConfigurer.java

@@ -279,6 +279,11 @@ public Object getProperty(String name) {
 			return super.source.getProperty(name);
 		}
 
+		@Override
+		public boolean containsProperty(String name) {
+			return super.source.containsProperty(name);
+		}
+
 		@Override
 		public String toString() {
 			return "FallbackEnvironmentPropertySource {environment=" + super.source + "}";