rhi: Prevent pipeline cache warning appearing again and again

As found by various users; one typical way to get these harmless
but annoying warnings reappearing again and again on every startup
is to run QML applications that only work with materials where the
shaders are conditioned so that the QRhi backend has no actual
"pipeline cache" data collected, meaning there is no data to write
out at the end, while at the same time have an existing cache file
generated from an older Qt or different graphics driver.

Pick-to: 6.5
Change-Id: Ie4deebc3ff60580a5435d55dd1869303f910bcb6
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

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