Fix "Return value of Tighten\SolanaPhpSdk\Program::__call() must be of the type array or null, int returned"

[miguilimzero]

If we try to use this code to get the current block value from node, we get error Return value of Tighten\SolanaPhpSdk\Program::__call() must be of the type array or null, int returned.

$client = new Connection(new SolanaRpcClient('api.mainnet-beta.solana.com'));
var_dump($client->getBlockHeight());

This occurr because the magic method expects the result to be an array or null, but int and strings can be returned as well depending on the RPC method called.

[mattstauffer]

Thanks @srdante!