[*] Backport of changes for task 7398.

Backport of changes for task 7398.

The following work has been done:

• Added dap_chain_addr_is_blank function to check if address is null or not. Now the dap_chain_addr_check_sum function returns true if the address is null. And the dap_chain_addr_from_str function can accept "null" then it will create an empty address. And dap_chain_addr_to_str will respectively return "null" if the address is empty.
• Made a couple of fixes to the staking code, now it passes an empty address instead of NULL.
• Made a correction to the function of creating a transaction output, now the function will not create a transaction if the address is specified as NULL and not only if the value is 0.