Fix fdo#33591 - edge-case trans gradients came out empty

Gradients with start==end got us zero steps, thus no fill at all.
Funnily this was wrong since pre-3.0 or somesuch.

drawinglayer/source/primitive2d/fillgradientprimitive2d.cxx

@@ -72,6 +72,8 @@ namespace drawinglayer
                 nSteps = nMaxSteps;
             }
 
+            nSteps = std::max(sal_uInt32(1), nSteps);
+
             switch(getFillGradient().getStyle())
             {
                 case attribute::GRADIENTSTYLE_LINEAR: