Fix no block device handling

Author: theamirocohen

features/storage/TESTS/blockdevice/general_block_device/README.md

@@ -32,7 +32,7 @@ mbed test -m K64F -t ARM -n mbed-os-features-storage-tests-blockdevice-general_b
 Use `mbed test` again:
 
 ```
-mbed test -m K64F -t ARM -n mbed-os-features-storage-tests-blockdevice-general_block_device --run
+mbed test -m K64F -t ARM -n mbed-os-features-storage-tests-blockdevice-general_block_device --run -v
 ```
 
 #### Troubleshooting

features/storage/TESTS/blockdevice/general_block_device/main.cpp

@@ -95,6 +95,11 @@ void test_random_program_read_erase()
 
     BlockDevice *block_device = BlockDevice::get_default_instance();
 
+    if (!block_device) {
+        utest_printf("\nno block device found.\n");
+        return;
+    }
+
     int err = block_device->init();
     TEST_ASSERT_EQUAL(0, err);
 
@@ -164,6 +169,11 @@ void test_multi_threads()
 
     BlockDevice *block_device = BlockDevice::get_default_instance();
 
+    if (!block_device) {
+        utest_printf("\nno block device found.\n");
+        return;
+    }
+
     int err = block_device->init();
     TEST_ASSERT_EQUAL(0, err);