[backport] don't try to dissolve buffer results if there are no features (fix #46396)

[alexbruy]

Description

Buffering an empty layer with native "Buffer" algorithm produces empty output layer, while the same operation with enabled "Dissolve result" option ends with error message

Feature could not be written to Buffered_a03cb901_262a_4ed5_b22c_23aeb10000b1: Could not add feature with geometry type GeometryCollection to layer of type MultiPolygon
Could not write feature into OUTPUT
Execution failed after 0.01 seconds

This creates problems when algorithm is used as part of a model. For consistence it is better to produce empty layer in both cases. Fixes #46396.

Manual backport of #47348.