Do not offer to start a call in a DM with no other member

A DM whose other member has left still offered the call button. Starting
that call put the user alone in a room nobody could join, with no way out
of the call screen.

The presenter now clears canStartCall for a DM with no other active
member. activeMembersCount counts joined and invited members, so a fresh
DM whose peer has been invited but has not joined yet can still start a
call. Joining an already ongoing call is untouched, so a call started
before the other member left stays reachable.

Relates to https://github.com/element-hq/element-x-android/issues/6189

features/roomcall/impl/src/main/kotlin/io/element/android/features/roomcall/impl/RoomCallStatePresenter.kt

@@ -60,10 +60,18 @@ class RoomCallStatePresenter(
                         isUserLocallyInTheCall = isUserLocallyInTheCall,
                         isAudioCall = roomInfo.activeCallIntentConsensus.isAudio(),
                     )
-                    else -> RoomCallState.StandBy(
-                        canStartCall = canJoinCall,
-                        isDM = roomInfo.isDm
-                    )
+                    else -> {
+                        // Nobody would ever join a call started in a DM whose other member has left,
+                        // and the call screen has no way out of that state.
+                        // activeMembersCount counts joined and invited members, so a DM whose peer
+                        // has been invited but has not joined yet can still start a call.
+                        // See https://github.com/element-hq/element-x-android/issues/6189
+                        val isEmptyDm = roomInfo.isDm && roomInfo.activeMembersCount <= 1
+                        RoomCallState.StandBy(
+                            canStartCall = canJoinCall && !isEmptyDm,
+                            isDM = roomInfo.isDm
+                        )
+                    }
                 }
             }
         }

features/roomcall/impl/src/test/kotlin/io/element/android/features/roomcall/impl/RoomCallStatePresenterTest.kt

@@ -49,6 +49,42 @@ class RoomCallStatePresenterTest {
         }
     }
 
+    @Test
+    fun `present - a DM with no other member cannot start a call`() = runTest {
+        val room = FakeJoinedRoom(
+            baseRoom = FakeBaseRoom(
+                initialRoomInfo = aRoomInfo(isDm = true, activeMembersCount = 1, joinedMembersCount = 1),
+                roomPermissions = roomPermissions(true),
+            )
+        )
+        val presenter = createRoomCallStatePresenter(joinedRoom = room)
+        presenter.test {
+            skipItems(1)
+            assertThat(awaitItem()).isEqualTo(
+                RoomCallState.StandBy(
+                    canStartCall = false,
+                    isDM = true
+                )
+            )
+        }
+    }
+
+    @Test
+    fun `present - a DM becomes callable once the other member is active`() = runTest {
+        val baseRoom = FakeBaseRoom(
+            initialRoomInfo = aRoomInfo(isDm = true, activeMembersCount = 1, joinedMembersCount = 1),
+            roomPermissions = roomPermissions(true),
+        )
+        val room = FakeJoinedRoom(baseRoom = baseRoom)
+        val presenter = createRoomCallStatePresenter(joinedRoom = room)
+        presenter.test {
+            skipItems(1)
+            assertThat(awaitItem()).isEqualTo(RoomCallState.StandBy(canStartCall = false, isDM = true))
+            baseRoom.givenRoomInfo(aRoomInfo(isDm = true, activeMembersCount = 2, joinedMembersCount = 2))
+            assertThat(awaitItem()).isEqualTo(RoomCallState.StandBy(canStartCall = true, isDM = true))
+        }
+    }
+
     @Test
     fun `present - element call not available`() = runTest {
         val room = FakeJoinedRoom(