Don't allow empty functions for before/after install

If you invoke fpm with --before-upgrade, but not --before-install, then
the resultant before_install function becomes an empty function, which
bash/dash do not allow. (Syntax errors) This same problem applies to the
after flags. To avoid this, we'll insert a null statement if there's no
install script specified.

We don't need to do this for the (before|after)_upgrade functions, since
the time these templates are used is when --(before|after)-upgrade is
specified.

templates/deb/postinst_upgrade.sh.erb

@@ -8,6 +8,8 @@ after_upgrade() {
 after_install() {
 <% if script?(:after_install) -%>
 <%=  script(:after_install) %>
+<% else -%>
+  :
 <% end -%>
 }
 

templates/deb/preinst_upgrade.sh.erb

@@ -8,6 +8,8 @@ before_upgrade() {
 before_install() {
 <% if script?(:before_install) -%>
 <%=  script(:before_install) %>
+<% else -%>
+  :
 <% end -%>
 }