keymaps: Update no action bindings to null (#54506)

Update uses of `zed::NoAction` in default keymaps with `null` seeing as
`zed::NoAction` is being deprecated.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -748,7 +748,9 @@
   {
     "context": "Workspace && debugger_running",
     "bindings": {
-      "f5": "zed::NoAction",
+      // Unbind `f5` to avoid dispatching the `debugger::Rerun` as that is what `f5` is bound to in the `Workspace`
+      // context.
+      "f5": null,
     },
   },
   {

assets/keymaps/default-macos.json

@@ -805,7 +805,9 @@
     "context": "Workspace && debugger_running",
     "use_key_equivalents": true,
     "bindings": {
-      "f5": "zed::NoAction",
+      // Unbind `f5` to avoid dispatching the `debugger::Rerun` as that is what `f5` is bound to in the `Workspace`
+      // context.
+      "f5": null,
       "f11": "debugger::StepInto",
     },
   },

assets/keymaps/default-windows.json

@@ -706,7 +706,9 @@
     "context": "Workspace && debugger_running",
     "use_key_equivalents": true,
     "bindings": {
-      "f5": "zed::NoAction",
+      // Unbind `f5` to avoid dispatching the `debugger::Rerun` as that is what `f5` is bound to in the `Workspace`
+      // context.
+      "f5": null,
     },
   },
   {