Fix #139177: Realize Instances Node results in empty curves

Regression in [0] caused by new curve data blocks being created without
"ob_type" being set.

[0]: 8fea423e00

source/blender/makesdna/DNA_curve_defaults.h

@@ -38,6 +38,8 @@
     .bevresol = 4, \
     .bevel_mode = CU_BEV_MODE_ROUND, \
     .taper_radius_mode = CU_TAPER_RADIUS_OVERRIDE, \
+    /* Use a default to prevent invalid values. */ \
+    .ob_type = OB_CURVES_LEGACY, \
   }
 
 /** \} */