Fixed a bug in the HSTS module around null headers

A colleague discovered that if an HTTPS server did not return any headers, this module would fail.

An example would be:

    $ curl -I -k https://11.22.33.44
    curl: (52) Empty reply from server

This would cause the error in the module:

    msf auxiliary(http_hsts) > rexploit
    [*] Reloading module...
    
    [*] 11.22.33.44:443: 
    [-] Auxiliary failed: NoMethodError undefined method `headers' for nil:NilClass
    [-] Call stack:
    [-]   /usr/local/share/metasploit-framework/modules/auxiliary/scanner/http/http_hsts.rb:39:in `run_host'
    [-]   /usr/local/share/metasploit-framework/lib/msf/core/auxiliary/scanner.rb:94:in `block in run'
    [-]   /usr/local/share/metasploit-framework/lib/msf/core/thread_manager.rb:100:in `call'
    [-]   /usr/local/share/metasploit-framework/lib/msf/core/thread_manager.rb:100:in `block in spawn'
    [*] Auxiliary module execution completed

To fix this, I just made sure to check that the response was not nil before trying to read the HSTS header.