Make Show.fromToString null safe

Fixes #4689

Author: rossabaker

core/src/main/scala/cats/Show.scala

@@ -68,7 +68,7 @@ object Show extends ScalaVersionSpecificShowInstances with ShowInstances {
   /**
    * creates an instance of [[Show]] using object toString
    */
-  def fromToString[A]: Show[A] = _.toString
+  def fromToString[A]: Show[A] = String.valueOf(_)
 
   final case class Shown(override val toString: String) extends AnyVal
   object Shown {

tests/shared/src/test/scala/cats/tests/ShowSuite.scala

@@ -113,6 +113,17 @@ class ShowSuite extends CatsSuite {
     val mapSorted: SortedMap[Int, String] = SortedMap(3 -> "three", 1 -> "one", 4 -> "four", 2 -> "two")
     assertEquals(show"$mapSorted", "SortedMap(1 -> one, 2 -> two, 3 -> three, 4 -> four)")
   }
+
+  test("fromToString should be consistent with toString on non-null references") {
+    case class ExplicitToString() { override val toString = "explicit toString" }
+    val show = Show.fromToString[ExplicitToString]
+    assertEquals(show.show(ExplicitToString()), "explicit toString")
+  }
+
+  test("fromToString should be null safe") {
+    val show = Show.fromToString[AnyRef]
+    assertEquals(show.show(null), "null")
+  }
 }
 
 final class ShowSuite2 extends munit.FunSuite {