3.0.1: Properly format change URL when patch set number is missing

If the patch set number was missing, the text "null" was appearing in
the URL rather than "" as was intended. This change addresses that and
also drops the trailing "/" if patch set is missing (which should be
considered equivalent by Gerrit)

app/build.gradle

@@ -1,5 +1,5 @@
 group 'sims.michael.gerritslackbot'
-version '2.3.1'
+version '3.0.1'
 
 buildscript {
     ext.kotlin_version = '1.2.20'

app/src/main/java/sims/michael/gerritslackbot/slack/gerrit-urls.kt

@@ -16,4 +16,4 @@ private fun formatGerritBaseUrlUsingHttpUrl(changeUrl: String): String? {
 }
 
 fun formatGerritChangeUrl(baseUrl: String, changeNumber: Int, patchSetNumber: Int?): String?
-        = "$baseUrl/$changeNumber/$patchSetNumber"
+        = listOfNotNull(baseUrl, changeNumber, patchSetNumber).joinToString("/")

tests/src/test/java/sims/michael/gerritslackbot/slack/GerritUrlsTest.kt

@@ -23,4 +23,9 @@ class GerritUrlsTest {
     fun formatGerritChangeUrl_works_as_expected() {
         assertEquals("http://server:8976/1/2", formatGerritChangeUrl("http://server:8976", 1, 2))
     }
+
+    @Test
+    fun formatGerritChangeUrl_handles_null_patchSetNumber_as_expected() {
+        assertEquals("http://server:8976/1", formatGerritChangeUrl("http://server:8976", 1, null))
+    }
 }