How to draw curve arrow with empty body?

One easy no-tricks solution: Draw some guides and draw an arrow with them:

1. Draw 2 circles and a line ,duplicate the line and flip the duplicate. Align all horizontally and vertically

2. Select the circles, apply to them Path > Object to Path. Apply Extensions > Generate from Path > Interpolate > 3 steps to get equally spaced three intermediate circles

3. Have all snap to points ON. Draw the arrow by clicking 10 times with the Bezier curve tool (=the Pen) to get the arrow as straight line segments. The drawing tool snaps on crossings and paths.

4-5. Select the node tool. Hold shift and drag the Bezier handles of the middle nodes out of the node to get the roundness. The result is shown in 5 without the guide shapes.

6. Make a vertically and horizontally flipped copy if needed.

The method can be criticized “it’s not exact, the curvature is eyeballed”. True, but it can still be ok for illustration purposes. If you need exact curvature you can for example cut parts of circles or draw circular arcs. I tried to avoid messing with separate arcs and lines or triangles.

You can combine a triangle and a circular arc. I cannot recommend to use arrowheads because they behave in a complex way in Inkscape. A triangle is much easier to control.

The parts must be drawn so that the fitting edges are parallel. In the left there’s a triangle drawn with the star-polygon tool. Hold Ctrl-key to get it horizontal. Below there’s a vertically squeezed copy which very likely is a better arrowhead.

In the right there’s a circular arc. Its starting angle is 0 degrees to make it fit with the horizontal triangle. Hold again Ctrl. It makes the arc circular as you draw. The next steps:

1. Place the triangle. It snaps to the top of the arc if you have all point snaps ON

2. Adjust the stroke width of the arc. Apply Path > Stroke to path when you are ready. You get a filled closed path.

3. Select the triangle and the arc. Apply Path > Union. Remove the fill color and set the wanted stroke color and style.

4. Flip or rotate the arrow to the wanted position.