nvs_flash: Multi-page blob erased using nvs_erase_key should be cleaned properly

nvs_flash: Multi-page blob erased using nvs_erase_key should be cleaned properly

Earlier eraseItem function in Storage class would do lazy cleanup of
multi-page blobs if called using type "ANY" instead of "BLOB". It used to
just delete BLOB data and index would remain as is. Any subsequent read
would delete index entry as well. This however would return a valid
length without error if nvs_get_blob API was just used for finding
length and not reading the complete blob. This change fixes this issue.

Closes https://github.com/espressif/esp-idf/issues/3255