Fixed issue where endpoint would infinitely retry for a nullable resource

Recently a new method in Endpoints.kt called "getNullable" has been added. However, the execute method was not appropriately changed, leading to the method infinitely retrying the request when the result was null.

Closes #312

Author: Nielssg

src/commonMain/kotlin/com.adamratzman.spotify/http/Endpoints.kt

@@ -75,13 +75,12 @@ public abstract class SpotifyEndpoint(public val api: GenericSpotifyApi) {
         }
     }
 
-
     internal suspend fun get(url: String): String {
         return execute<String>(url)
     }
 
     internal suspend fun getNullable(url: String): String? {
-        return execute<String?>(url)
+        return execute<String?>(url, retryOnNull = false)
     }
 
     internal suspend fun post(url: String, body: String? = null, contentType: String? = null): String {
@@ -100,14 +99,16 @@ public abstract class SpotifyEndpoint(public val api: GenericSpotifyApi) {
         return execute<String>(url, body, HttpRequestMethod.DELETE, contentType = contentType)
     }
 
-    private suspend fun <T: String?> execute(
+    @Suppress("UNCHECKED_CAST")
+    private suspend fun <ReturnType: String?> execute(
         url: String,
         body: String? = null,
         method: HttpRequestMethod = HttpRequestMethod.GET,
         retry202: Boolean = true,
         contentType: String? = null,
-        attemptedRefresh: Boolean = false
-    ): String {
+        attemptedRefresh: Boolean = false,
+        retryOnNull: Boolean = true
+    ): ReturnType {
         if (api.token.shouldRefresh()) {
             if (!api.spotifyApiOptions.automaticRefresh) throw SpotifyException.ReAuthenticationNeededException(message = "The access token has expired.")
             else api.refreshToken()
@@ -116,7 +117,7 @@ public abstract class SpotifyEndpoint(public val api: GenericSpotifyApi) {
         val spotifyRequest = SpotifyRequest(url, method, body, api)
         val cacheState = if (api.useCache) cache[spotifyRequest] else null
 
-        if (cacheState?.isStillValid() == true) return cacheState.data
+        if (cacheState?.isStillValid() == true) return cacheState.data as ReturnType
         else if (cacheState?.let { it.eTag == null } == true) {
             cache -= spotifyRequest
         }
@@ -131,18 +132,18 @@ public abstract class SpotifyEndpoint(public val api: GenericSpotifyApi) {
                         retryIfInternalServerErrorLeft = api.spotifyApiOptions.retryOnInternalServerErrorTimes
                     )
 
-                    handleResponse(document, cacheState, spotifyRequest, retry202) ?: execute<T>(
-                        url,
-                        body,
-                        method,
-                        false,
-                        contentType
-                    )
+                    handleResponse(document, cacheState, spotifyRequest, retry202) ?: run {
+                        if (retryOnNull) {
+                            execute<ReturnType>(url, body, method, false, contentType)
+                        } else {
+                            null
+                        }
+                    }
                 } catch (e: BadRequestException) {
                     if (e.statusCode == 401 && !attemptedRefresh) {
                         api.refreshToken()
 
-                        execute<T>(
+                        execute<ReturnType>(
                             url,
                             body,
                             method,
@@ -152,7 +153,7 @@ public abstract class SpotifyEndpoint(public val api: GenericSpotifyApi) {
                         )
                     } else throw e
                 }
-            }
+            } as ReturnType
         } catch (e: CancellationException) {
             throw TimeoutException(
                 e.message