Add null safety to glossary

src/data/glossary.yml

@@ -291,6 +291,33 @@
     - "design"
     - "ui"
 
+- term: "Null safety"
+  short_description: |-
+    A Dart feature that prevents errors that result from accessing variables set to `null`.
+  long_description: |-
+    Dart's null safety prevents errors that result from unintentional access
+    of variables set to `null`.
+
+    With [sound null safety][], variables are non-nullable by default:
+    they can only be assigned a value of `null` if you explicitly declare
+    them as nullable.
+    This differs from other "mixed" null safety implementations,
+    where a non-nullable variable could still contain `null` at runtime.
+    With Dart's sound null safety, the compiler guarantees that a
+    non-nullable variable can never be `null`.
+
+    [sound null safety]: https://dart.dev/null-safety
+  related_links:
+    - text: "Sound null safety"
+      link: "https://dart.dev/null-safety"
+      type: "external"
+    - text: "Null safety codelab"
+      link: "https://dart.dev/codelabs/null-safety"
+      type: "tutorial"
+  labels:
+    - "language"
+    - "fundamentals"
+
 - term: "Prop drilling"
   short_description: |-
     The process of passing data through multiple layers of widgets