fix segmentation fault on nil slice element

testdata/ignore_nil_elements/cmds/Taskfile.yml

@@ -0,0 +1,7 @@
+version: '3'
+
+tasks:
+  default:
+    cmds:
+      - echo "string-slice-1"
+      -

testdata/ignore_nil_elements/deps/Taskfile.yml

@@ -0,0 +1,11 @@
+version: '3'
+
+tasks:
+  default:
+    deps:
+      -
+      - task: dep
+
+  dep:
+    cmds:
+      - echo "string-slice-1"

testdata/ignore_nil_elements/preconditions/Taskfile.yml

@@ -0,0 +1,9 @@
+version: '3'
+
+tasks:
+  default:
+    preconditions:
+      -
+      - sh: "[ 1 = 1 ]"
+    cmds:
+      - echo "string-slice-1"