Fix Telegram bridge proxying config when matrix-nginx-proxy disabled

Related to #189 (Github Issue).

People had proxying problems if:
- they used the whole playbook (including the `matrix-nginx-proxy` role)
- and they were disabling the proxy (`matrix_nginx_proxy_enabled: false`)
- and they were proxying with their own nginx server

For them,
`matrix_nginx_proxy_proxy_matrix_additional_server_configuration_blocks`
would not be modified to inject the necessary proxying configuration.

roles/matrix-bridge-mautrix-telegram/tasks/setup_install.yml

@@ -94,18 +94,18 @@
 - block:
   - name: Fail if matrix-nginx-proxy role already executed
     fail:
-      msg: >
+      msg: >-
         Trying to append Mautrix Telegram's reverse-proxying configuration to matrix-nginx-proxy,
         but it's pointless since the matrix-nginx-proxy role had already executed.
         To fix this, please change the order of roles in your plabook,
         so that the matrix-nginx-proxy role would run after the matrix-bridge-mautrix-telegram role.
-    when: matrix_nginx_proxy_role_executed|bool
+    when: matrix_nginx_proxy_role_executed|default(False)|bool
 
   - name: Generate Mautrix Telegram proxying configuration for matrix-nginx-proxy
     set_fact:
       matrix_mautrix_telegram_matrix_nginx_proxy_configuration: |
         location {{ matrix_mautrix_telegram_public_endpoint }} {
-        {% if matrix_nginx_proxy_enabled %}
+        {% if matrix_nginx_proxy_enabled|default(False) %}
         	{# Use the embedded DNS resolver in Docker containers to discover the service #}
         	resolver 127.0.0.11 valid=5s;
         	set $backend "matrix-mautrix-telegram:8080";
@@ -120,20 +120,19 @@
     set_fact:
       matrix_nginx_proxy_proxy_matrix_additional_server_configuration_blocks: |
         {{
-          matrix_nginx_proxy_proxy_matrix_additional_server_configuration_blocks
+          matrix_nginx_proxy_proxy_matrix_additional_server_configuration_blocks|default([])
           +
           [matrix_mautrix_telegram_matrix_nginx_proxy_configuration]
         }}
-
-  when: "matrix_nginx_proxy_enabled|default(False)"
   tags:
    - always
 
 - name: Warn about reverse-proxying if matrix-nginx-proxy not used
   debug:
-    msg: >
+    msg: >-
       NOTE: You've enabled the Mautrix Telegram bridge but are not using the matrix-nginx-proxy
       reverse proxy.
       Please make sure that you're proxying the `{{ matrix_mautrix_telegram_public_endpoint }}`
       URL endpoint to the matrix-mautrix-telegram container.
+      You can expose the container's port using the `matrix_mautrix_telegram_container_http_host_bind_port` variable.
   when: "matrix_nginx_proxy_enabled is not defined"