internal/pkg: exec nil path check during cure

This results in os.ErrInvalid instead of a panic, which hopefully improves user experience.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/exec_test.go

@@ -96,6 +96,18 @@ func TestExec(t *testing.T) {
 						},
 					}),
 				), nil, pkg.Checksum{}, errors.Join(stub.UniqueError(0xcafe))},
+
+				{"invalid paths", pkg.NewExec(
+					t.Context(),
+					msg,
+					0,
+					check.MustAbs("/work"),
+					[]string{"HAKUREI_TEST=1"},
+					check.MustAbs("/opt/bin/testtool"),
+					[]string{"testtool"},
+
+					pkg.ExecContainerPath{},
+				), nil, pkg.Checksum{}, os.ErrInvalid},
 			})
 
 			// check init failure passthrough