Use g_return_val_if_fail / g_return_if_fail for NULL guards

Replace verbose if-block NULL checks with idiomatic GLib
precondition macros, per review feedback.

libgnucash/app-utils/gnc-gsettings.cpp

@@ -105,20 +105,10 @@ static GSettings * gnc_gsettings_get_settings_obj (const gchar *schema_str)
     auto full_name_str = normalize_schema_name (schema_str);
     auto full_name = full_name_str.c_str();
     auto schema_source {g_settings_schema_source_get_default()};
-    if (!schema_source)
-    {
-        PWARN ("No GSettings schema source available; cannot access schema %s", full_name);
-        LEAVE("");
-        return nullptr;
-    }
+    g_return_val_if_fail(schema_source, nullptr);
 
     auto schema {g_settings_schema_source_lookup(schema_source, full_name, true)};
-    if (!schema)
-    {
-        PWARN ("GSettings schema %s not found", full_name);
-        LEAVE("");
-        return nullptr;
-    }
+    g_return_val_if_fail(schema, nullptr);
 
     auto gset = g_settings_new_full (schema, nullptr, nullptr);
     DEBUG ("Created gsettings object %p for schema %s", gset, full_name);
@@ -559,11 +549,7 @@ gnc_settings_dump_schema_paths (void)
     gchar **non_relocatable;
 
     auto schema_source {g_settings_schema_source_get_default()};
-    if (!schema_source)
-    {
-        PWARN ("No GSettings schema source available; cannot dump schema paths");
-        return;
-    }
+    g_return_if_fail(schema_source);
 
     g_settings_schema_source_list_schemas (schema_source, true,
                                            &non_relocatable, nullptr);