fix: Emit TLS error if there are no certs available

Before, if neither the vhost-specific cert nor `default.crt` existed,
nginx-proxy would not create the https vhost.  This resulted in nginx
either refusing the connection or serving the wrong vhost depending on
whether there was another https vhost with a certificate.

Now nginx-proxy always creates an https server for a vhost, even if
the vhost-specific certificate and the default certificate are both
missing.  When both certs are missing, nginx is given empty
certificate data to make it possible for it to start up without an
error.  The empty certificate data causes the user to see a TLS error,
which is much easier to troubleshoot than a connection refused error
or serving the wrong vhost.

test/test_fallback.data/nodefault.yml

@@ -0,0 +1,36 @@
+services:
+  sut:
+    image: nginxproxy/nginx-proxy:test
+    volumes:
+      - /var/run/docker.sock:/tmp/docker.sock:ro
+      - ./nodefault.certs:/etc/nginx/certs:ro
+  https-and-http:
+    image: web
+    expose:
+      - "81"
+    environment:
+      WEB_PORTS: "81"
+      VIRTUAL_HOST: https-and-http.nginx-proxy.test
+  https-only:
+    image: web
+    expose:
+      - "82"
+    environment:
+      WEB_PORTS: "82"
+      VIRTUAL_HOST: https-only.nginx-proxy.test
+      HTTPS_METHOD: nohttp
+  http-only:
+    image: web
+    expose:
+      - "83"
+    environment:
+      WEB_PORTS: "83"
+      VIRTUAL_HOST: http-only.nginx-proxy.test
+      HTTPS_METHOD: nohttps
+  missing-cert:
+    image: web
+    expose:
+      - "84"
+    environment:
+      WEB_PORTS: "84"
+      VIRTUAL_HOST: missing-cert.nginx-proxy.test