Remove empty steps when decoding Job (#49)

Follow #48
Empty steps are invalid, so remove them when decoding `Job` from YAML.

Reviewed-on: https://gitea.com/gitea/act/pulls/49
Reviewed-by: Jason Song <[email protected]>
Co-authored-by: Zettat123 <[email protected]>
Co-committed-by: Zettat123 <[email protected]>

pkg/jobparser/testdata/empty_step.in.yaml

@@ -0,0 +1,8 @@
+name: test
+jobs:
+  job1:
+    name: job1
+    runs-on: linux
+    steps:
+      - run: echo job-1
+      -

pkg/jobparser/testdata/empty_step.out.yaml

@@ -0,0 +1,7 @@
+name: test
+jobs:
+  job1:
+    name: job1
+    runs-on: linux
+    steps:
+      - run: echo job-1