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 <i@wolfogre.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-committed-by: Zettat123 <zettat123@gmail.com>

act/jobparser/jobparser_test.go

@@ -42,6 +42,11 @@ func TestParse(t *testing.T) {
 			options: nil,
 			wantErr: false,
 		},
+		{
+			name:    "empty_step",
+			options: nil,
+			wantErr: false,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {