[FixRM #8494] Don't try to create war without exe

History here: https://dev.metasploit.com/redmine/issues/8494

* Verification 

- [x] Before patch:

```
$ ./msfpayload solaris/sparc/shell_reverse_tcp LHOST=1.2.3.4 W > /tmp/test.war
/Users/juan/Projects/git/metasploit-framework/lib/msf/util/exe.rb:1055:in `to_jsp_war': undefined method `unpack' for nil:NilClass (NoMethodError)
	from ./msfpayload:219:in `<main>'
```

- [x] After patch:

```
$ ./msfpayload solaris/sparc/shell_reverse_tcp LHOST=1.2.3.4 W > /tmp/test.war
No executable format support for this arch/platform
```