[BUG] Fresh install of Mirador with no enabled plugins emits PHP Warning

[kayakr]

What steps does it take to reproduce the issue?

• Enable Islandora Mirador 2.3.5 and render via Mirador block. Observe 2 instances of "Warning: Undefined array key "miradorImageToolsPlugin" in template_preprocess_mirador() (line 51 of modules/contrib/islandora_mirador/islandora_mirador.module).
• Enable Islandora Mirador 2.3.5 and render via Mirador block. Observe 2 instances of "Warning: Undefined array key "textOverlayPlugin" in template_preprocess_mirador() (line 51 of modules/contrib/islandora_mirador/islandora_mirador.module).

On L54 of Mirador module https://github.com/Islandora/islandora_mirador/blob/2.x/islandora_mirador.module#L54, need to add isset(), e.g. if (isset($enabled_plugins[$plugin_id])) {