docs(sample): Table exists sample fix (#1868)

Added `null` check in the sample and modified the test case to test for "Table not found"

Author: prash-mi

samples/snippets/src/main/java/com/example/bigquery/TableExists.java

@@ -40,7 +40,10 @@ public static void tableExists(String datasetName, String tableName) {
       BigQuery bigquery = BigQueryOptions.getDefaultInstance().getService();
 
       Table table = bigquery.getTable(TableId.of(datasetName, tableName));
-      if (table.exists()) {
+      if (table != null
+          && table
+              .exists()) { // table will be null if it is not found and setThrowNotFound is not set
+        // to `true`
         System.out.println("Table already exist");
       } else {
         System.out.println("Table not found");

samples/snippets/src/test/java/com/example/bigquery/TableExistsIT.java

@@ -61,7 +61,6 @@ public void setUp() {
     System.setOut(out);
     // create a temporary table
     tableName = "MY_TABLE_NAME_TEST_" + UUID.randomUUID().toString().substring(0, 8);
-    CreateTable.createTable(BIGQUERY_DATASET_NAME, tableName, Schema.of());
   }
 
   @After
@@ -76,6 +75,9 @@ public void tearDown() {
 
   @Test
   public void testTableExists() {
+    TableExists.tableExists(BIGQUERY_DATASET_NAME, tableName);
+    assertThat(bout.toString()).contains("Table not found");
+    CreateTable.createTable(BIGQUERY_DATASET_NAME, tableName, Schema.of());
     TableExists.tableExists(BIGQUERY_DATASET_NAME, tableName);
     assertThat(bout.toString()).contains("Table already exist");
   }