In Oracle ignore non-existant tables when dropping

If tables don't exist, ignore that error when dropping.

This aligns Oracle with the existing mysql, postgresql, h2, and hsqldb behavior.

spring-session-jdbc/src/main/resources/org/springframework/session/jdbc/schema-drop-oracle.sql

@@ -1,2 +1,18 @@
-DROP TABLE SPRING_SESSION_ATTRIBUTES;
-DROP TABLE SPRING_SESSION;
+BEGIN
+	BEGIN
+		EXECUTE IMMEDIATE 'DROP TABLE SPRING_SESSION_ATTRIBUTES';
+	EXCEPTION
+		WHEN OTHERS THEN
+			IF SQLCODE != -942 THEN
+				RAISE;
+			END IF;
+	END;
+	BEGIN
+		EXECUTE IMMEDIATE 'DROP TABLE SPRING_SESSION';
+	EXCEPTION
+		WHEN OTHERS THEN
+			IF SQLCODE != -942 THEN
+				RAISE;
+			END IF;
+	END;
+END;