Fix creation or animation of a zero-length DashedLine (#4606)

* fixing #4591

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

manim/mobject/geometry/arc.py

@@ -241,7 +241,8 @@ class TipableVMobject(VMobject, metaclass=ConvertToOpenGL):
         if self.has_start_tip():
             result.add(self.start_tip)
             self.remove(self.start_tip)
-        self.put_start_and_end_on(start, end)
+        if result.submobjects:
+            self.put_start_and_end_on(start, end)
         return result
 
     def get_tips(self) -> VGroup:

tests/module/mobject/types/vectorized_mobject/test_dashed_vmobject.py

@@ -1,4 +1,4 @@
-from manim import ORIGIN, UR, Arrow, DashedVMobject, VGroup
+from manim import ORIGIN, UR, Arrow, DashedLine, DashedVMobject, VGroup
 from manim.mobject.geometry.tips import ArrowTip, StealthTip
 
 
@@ -41,3 +41,19 @@ def test_dashed_arrow_with_start_tip_has_two_tips():
     tips = _collect_tips(dashed)
 
     assert len(tips) == 2
+
+
+def test_zero_length_dashed_line_submobjects_have_2d_points():
+    """Submobjects of a zero-length DashedLine must have 2-D point arrays."""
+    line = DashedLine(ORIGIN, ORIGIN)
+    for sub in line.submobjects:
+        assert sub.points.ndim == 2, (
+            f"Expected 2-D points array, got shape {sub.points.shape}"
+        )
+
+
+def test_become_nonzero_to_zero_dashed_line_does_not_crash():
+    """become() from a normal DashedLine to a zero-length one should not crash."""
+    normal = DashedLine(ORIGIN, 2 * UR)
+    zero = DashedLine(ORIGIN, ORIGIN)
+    normal.become(zero)