QGIS crashes with QgsGeometry method on empty collection

[uclaros]

Describe the bug
QGIS crashes when calling QgsGeometry::nearestPoint() on an empty collection QgsGeometry

How to Reproduce
On a python console run the following:

g=QgsGeometry.fromWkt('GeometryCollection EMPTY')
g.nearestPoint(QgsGeometry.fromWkt('Point (42  0)'))

QGIS and OS versions
81911c1 on Debian testing (geos 3.8.1) , 3.10.2 on windows (geos 3.8.0)

Additional context
the empty geometry collection also gives the following strange result:
Nanananananananan, Batman!

>>> QgsGeometry.fromWkt('GeometryCollection EMPTY').minimalEnclosingCircle()

(<QgsGeometry: Polygon ((nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan, nan nan))>, <QgsPointXY: POINT EMPTY>, 0.0)

[uclaros]

@gioman This is not a PyQGIS issue, the python snippet is just for easy reproduction. QgsGeos is probably responsible.

[obrix]

I can reproduce those two bugs :

• For the first one it seems to be an issue with the use of an invalid GEOSNearestPoints_r result. Should be fixed by avoiding calling it when the geometry is empty.

• Second one comes from the fact minimalEnclosingCircle has an hidden default parameters for the size of the number of segments of the enclosing circle (36). What you see is a default invalid result for the minimalEnclosingCircle algorithm : an invalid 36 segments polygon . In this case I think it is preferable to also return a null geometry if the geometry on which we call the algorithm is empty.