Duck.ai: Provide default value for tabs (#7129)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/1157893581871903/task/1211962674281407?focus=true

### Description
This PR fixes the access to the tabs when null

### Steps to test this PR

_Third party open_
- [x] Install the app and make it default browser
- [x] Enable Duck.ai and force close the app
- [x] Open DDG via a third party app (Reddit for example)
- [x] Verify it works as expected

Author: malmstein

app/src/main/java/com/duckduckgo/app/browser/BrowserViewModel.kt

@@ -509,7 +509,8 @@ class BrowserViewModel @Inject constructor(
     ) {
         val duckAiFullScreenMode = duckAiFeatureState.showFullScreenMode.value
         logcat(INFO) { "Duck.ai openDuckChat duckChatSessionActive $duckChatSessionActive" }
-        sendCommand(OpenDuckChat(duckChatUrl, duckChatSessionActive, withTransition, tabs.value.size, duckAiFullScreenMode))
+        val tabsCount = tabs.value?.size ?: 0
+        sendCommand(OpenDuckChat(duckChatUrl, duckChatSessionActive, withTransition, tabsCount, duckAiFullScreenMode))
     }
 }
 

app/src/test/java/com/duckduckgo/app/browser/BrowserViewModelTest.kt

@@ -612,6 +612,39 @@ class BrowserViewModelTest {
         }
     }
 
+    @Test
+    fun `when openDuckChat called and tabs are null then command is sent with 0 tabs`() = runTest {
+        doReturn(MutableLiveData(null)).whenever(mockTabRepository).liveTabs
+        initTestee()
+
+        testee.openDuckChat(duckChatUrl = "duck://chat", duckChatSessionActive = false, withTransition = false)
+
+        testee.commands.test {
+            val command = awaitItem()
+            assertTrue(command is Command.OpenDuckChat)
+            command as Command.OpenDuckChat
+            assertEquals(0, command.tabs)
+            cancelAndIgnoreRemainingEvents()
+        }
+    }
+
+    @Test
+    fun `when openDuckChat called then command is sent with correct tab count`() = runTest {
+        val tabs = listOf(TabEntity("1", "", "", position = 0))
+        doReturn(MutableLiveData(tabs)).whenever(mockTabRepository).liveTabs
+        initTestee()
+
+        testee.openDuckChat(duckChatUrl = "duck://chat", duckChatSessionActive = false, withTransition = false)
+
+        testee.commands.test {
+            val command = awaitItem()
+            assertTrue(command is Command.OpenDuckChat)
+            command as Command.OpenDuckChat
+            assertEquals(tabs.size, command.tabs)
+            cancelAndIgnoreRemainingEvents()
+        }
+    }
+
     private fun initTestee() {
         testee = BrowserViewModel(
             tabRepository = mockTabRepository,