entity: Instance: fix optional pleroma field type

app/src/main/java/com/keylesspalace/tusky/entity/Instance.kt

@@ -31,7 +31,7 @@ data class Instance (
         @SerializedName("max_toot_chars") val maxTootChars: Int?,
         @SerializedName("max_bio_chars") val maxBioChars: Int?,
         @SerializedName("poll_limits") val pollLimits: PollLimits?,
-        val pleroma: InstancePleroma
+        val pleroma: InstancePleroma?
 ) {
     override fun hashCode(): Int {
         return uri.hashCode()

app/src/test/java/com/keylesspalace/tusky/ComposeActivityTest.kt

@@ -503,6 +503,7 @@ class ComposeActivityTest {
                 ),
                 maximumTootCharacters,
                 null,
+                null,
                 null
         )
     }