Add null-safe SharedPreferences.getStringSafe

Null-safe alternative to SharedPreferences.getString that guarantees the return value is non-null when defValue is non-null.

app/src/main/java/org/schabi/newpipe/ktx/SharedPreferences.kt

@@ -0,0 +1,7 @@
+package org.schabi.newpipe.ktx
+
+import android.content.SharedPreferences
+
+fun SharedPreferences.getStringSafe(key: String, defValue: String): String {
+    return getString(key, null) ?: defValue
+}