Improve isNullOrEmpty() extensions

- Annotate with SinceKotlin("1.3") and InlineOnly
- Add contract linking the receiver and the returned value
- Unify wording in docs
- Add sample for Map.isNullOrEmpty

#KT-23279

libraries/stdlib/samples/test/samples/collections/arrays.kt

@@ -17,6 +17,7 @@
 package samples.collections
 
 import samples.*
+import kotlin.test.*
 
 
 @RunWith(Enclosed::class)
@@ -38,10 +39,10 @@ class Arrays {
             val nullArray: Array<Any>? = null
             assertTrue(nullArray.isNullOrEmpty())
 
-            val emptyArray = emptyArray<Any>()
+            val emptyArray: Array<Any>? = emptyArray<Any>()
             assertTrue(emptyArray.isNullOrEmpty())
 
-            val array = arrayOf('a', 'b', 'c')
+            val array: Array<Char>? = arrayOf('a', 'b', 'c')
             assertFalse(array.isNullOrEmpty())
         }
     }

libraries/stdlib/samples/test/samples/collections/maps.kt

@@ -106,6 +106,18 @@ class Maps {
             }
         }
 
+
+        @Sample
+        fun mapIsNullOrEmpty() {
+            val nullMap: Map<String, Any>? = null
+            assertTrue(nullMap.isNullOrEmpty())
+
+            val emptyMap: Map<String, Any>? = emptyMap<String, Any>()
+            assertTrue(emptyMap.isNullOrEmpty())
+
+            val map: Map<Char, Int>? = mapOf('a' to 1, 'b' to 2, 'c' to 3)
+            assertFalse(map.isNullOrEmpty())
+        }
     }
 
     class Filtering {