Don't throw an error if a context is null in the DefaultAndroidResourceReader (#5433)

It fixes Robolectric tests where a context is available through
InstrumentationRegistry

Fixes https://youtrack.jetbrains.com/issue/CMP-6612

## Release Notes
### Fixes - Resources
- Fix resource access in a Robolectric test environment.

Author: terrakok

components/resources/library/src/androidMain/kotlin/org/jetbrains/compose/resources/ResourceReader.android.kt

@@ -13,13 +13,7 @@ internal actual fun getPlatformResourceReader(): ResourceReader = DefaultAndroid
 
 @ExperimentalResourceApi
 internal object DefaultAndroidResourceReader : ResourceReader {
-    private val assets: AssetManager by lazy {
-        val context = androidContext ?: error(
-            "Android context is not initialized. " +
-                    "If it happens in the Preview mode then call PreviewContextConfigurationEffect() function."
-        )
-        context.assets
-    }
+    private val assets: AssetManager? get() = androidContext?.assets
 
     private val instrumentedAssets: AssetManager?
         get() = try {
@@ -69,7 +63,7 @@ internal object DefaultAndroidResourceReader : ResourceReader {
             Uri.parse("file:///android_asset/$path")
         } else {
             val classLoader = getClassLoader()
-            val resource = classLoader.getResource(path) ?: throw MissingResourceException(path)
+            val resource = classLoader.getResource(path) ?: throwMissingResourceException(path)
             resource.toURI()
         }
         return uri.toString()
@@ -83,11 +77,23 @@ internal object DefaultAndroidResourceReader : ResourceReader {
                 instrumentedAssets.open(path)
             } catch (e: FileNotFoundException) {
                 val classLoader = getClassLoader()
-                classLoader.getResourceAsStream(path) ?: throw MissingResourceException(path)
+                classLoader.getResourceAsStream(path) ?: throwMissingResourceException(path)
             }
         }
     }
 
+    private fun throwMissingResourceException(path: String): Nothing {
+        if (androidContext == null) {
+            throw MissingResourceException(
+                path = path,
+                message = "Android context is not initialized. " +
+                    "If it happens in the Preview mode then call PreviewContextConfigurationEffect() function."
+            )
+        } else {
+            throw MissingResourceException(path)
+        }
+    }
+
     private fun getClassLoader(): ClassLoader {
         return this.javaClass.classLoader ?: error("Cannot find class loader")
     }

components/resources/library/src/commonMain/kotlin/org/jetbrains/compose/resources/ResourceReader.kt

@@ -4,7 +4,9 @@ import androidx.compose.runtime.Composable
 import androidx.compose.runtime.ProvidableCompositionLocal
 import androidx.compose.runtime.staticCompositionLocalOf
 
-class MissingResourceException(path: String) : Exception("Missing resource with path: $path")
+class MissingResourceException(path: String) : Exception("Missing resource with path: $path") {
+    internal constructor(path: String, message: String) : this("$path. $message")
+}
 
 /**
  * Reads the content of the resource file at the specified path and returns it as a byte array.