Handle null value and non existant value

In our project i have handled saving the data to a storage driver myself (added it well before it was included in the current package). 

When adding a test for this functionality in my project the latest version of the code throws an error about attempting to get a value from a null. 

I've seen you check for the values presence in other methods in this class, so i added it here too.

src/Concerns/IsSentEmailModel.php

@@ -98,11 +98,13 @@ public function getContentAttribute(): ?string
         if ($content = $this->attributes['content']) {
             return $content;
         }
-        if ($contentFilePath = $this->meta->get('content_file_path')) {
-            try {
-                return Storage::disk(config('mail-tracker.tracker-filesystem'))->get($contentFilePath);
-            } catch (FileNotFoundException $e) {
-                return null;
+        if ($this->meta?->has('content_file_path')) {
+            if ($contentFilePath = $this->meta->get('content_file_path')) {
+                try {
+                    return Storage::disk(config('mail-tracker.tracker-filesystem'))->get($contentFilePath);
+                } catch (FileNotFoundException $e) {
+                    return null;
+                }
             }
         }
         return null;
@@ -191,4 +193,4 @@ public function fillContent(string $originalHtml, string $hash)
 
         $this->content = $databaseContent;
     }
-}
+}