fix: return empty catalog name (#174)

* fix: return empty catalog name

The JDBC connection would return the current database name as the
current catalog name, but that catalog name would not be returned
in the ResultSet returned by getCatalogs(). This discrepancy causes
some tools to ignore or misinterpret the catalog and schema structure
of a Cloud Spanner database. Specifically, it breaks the autocomplete
feature of DBeaver.

* tests: fix failing test case

src/main/java/com/google/cloud/spanner/jdbc/JdbcConnection.java

@@ -291,7 +291,7 @@ public void setCatalog(String catalog) throws SQLException {
   @Override
   public String getCatalog() throws SQLException {
     checkClosed();
-    return getConnectionOptions().getDatabaseName();
+    return "";
   }
 
   @Override

src/test/java/com/google/cloud/spanner/jdbc/JdbcConnectionTest.java

@@ -634,7 +634,9 @@ public void testCatalog() throws SQLException {
     ConnectionOptions options = mock(ConnectionOptions.class);
     when(options.getDatabaseName()).thenReturn("test");
     try (JdbcConnection connection = createConnection(options)) {
-      assertThat(connection.getCatalog()).isEqualTo("test");
+      // The connection should always return the empty string as the current catalog, as no other
+      // catalogs exist in the INFORMATION_SCHEMA.
+      assertThat(connection.getCatalog()).isEqualTo("");
       // This should be allowed.
       connection.setCatalog("");
       try {