[SPARK-34770][SQL] InMemoryCatalog.tableExists should not fail if database doesn't exist

### What changes were proposed in this pull request?

This PR updates `InMemoryCatalog.tableExists` to return false if database doesn't exist, instead of failing. The new behavior is consistent with `HiveExternalCatalog` which is used in production, so this bug mostly only affects tests.

### Why are the changes needed?

bug fix

### Does this PR introduce _any_ user-facing change?

no

### How was this patch tested?

a new test

Closes #31860 from cloud-fan/catalog.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/InMemoryCatalog.scala

@@ -342,8 +342,7 @@ class InMemoryCatalog(
   }
 
   override def tableExists(db: String, table: String): Boolean = synchronized {
-    requireDbExists(db)
-    catalog(db).tables.contains(table)
+    catalog.contains(db) && catalog(db).tables.contains(table)
   }
 
   override def listTables(db: String): Seq[String] = synchronized {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalogSuite.scala

@@ -704,6 +704,9 @@ abstract class SessionCatalogSuite extends AnalysisTest with Eventually {
       catalog.createTempView("tbl3", tempTable, overrideIfExists = false)
       // tableExists should not check temp view.
       assert(!catalog.tableExists(TableIdentifier("tbl3")))
+
+      // If database doesn't exist, return false instead of failing.
+      assert(!catalog.tableExists(TableIdentifier("tbl1", Some("non-exist"))))
     }
   }