Fix not to ignore diff selector when it matched nothing

Fixes #327

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

pkg/app/testdata/app_diff_test/show_diff_on_changed_selected_release

@@ -0,0 +1,40 @@
+processing file "helmfile.yaml" in directory "."
+changing working directory to "/path/to"
+first-pass rendering starting for "helmfile.yaml.part.0": inherited=&{default map[] map[]}, overrode=<nil>
+first-pass uses: &{default map[] map[]}
+first-pass rendering output of "helmfile.yaml.part.0":
+ 0: 
+ 1: releases:
+ 2: - name: a
+ 3:   chart: incubator/raw
+ 4:   namespace: default
+ 5: - name: b
+ 6:   chart: incubator/raw
+ 7:   namespace: default
+ 8: 
+
+first-pass produced: &{default map[] map[]}
+first-pass rendering result of "helmfile.yaml.part.0": {default map[] map[]}
+vals:
+map[]
+defaultVals:[]
+second-pass rendering result of "helmfile.yaml.part.0":
+ 0: 
+ 1: releases:
+ 2: - name: a
+ 3:   chart: incubator/raw
+ 4:   namespace: default
+ 5: - name: b
+ 6:   chart: incubator/raw
+ 7:   namespace: default
+ 8: 
+
+merged environment: &{default map[] map[]}
+1 release(s) matching name=a found in helmfile.yaml
+
+processing 1 groups of releases in this order:
+GROUP RELEASES
+1     default/default/a
+
+processing releases in group 1/1: default/default/a
+changing working directory back to "/path/to"

pkg/app/testdata/app_diff_test/shows_no_diff_on_already_uninstalled_selected_release

@@ -0,0 +1,37 @@
+processing file "helmfile.yaml" in directory "."
+changing working directory to "/path/to"
+first-pass rendering starting for "helmfile.yaml.part.0": inherited=&{default map[] map[]}, overrode=<nil>
+first-pass uses: &{default map[] map[]}
+first-pass rendering output of "helmfile.yaml.part.0":
+ 0: 
+ 1: releases:
+ 2: - name: a
+ 3:   chart: incubator/raw
+ 4:   installed: false
+ 5:   namespace: default
+ 6: - name: b
+ 7:   chart: incubator/raw
+ 8:   namespace: default
+ 9: 
+
+first-pass produced: &{default map[] map[]}
+first-pass rendering result of "helmfile.yaml.part.0": {default map[] map[]}
+vals:
+map[]
+defaultVals:[]
+second-pass rendering result of "helmfile.yaml.part.0":
+ 0: 
+ 1: releases:
+ 2: - name: a
+ 3:   chart: incubator/raw
+ 4:   installed: false
+ 5:   namespace: default
+ 6: - name: b
+ 7:   chart: incubator/raw
+ 8:   namespace: default
+ 9: 
+
+merged environment: &{default map[] map[]}
+1 release(s) matching name=a found in helmfile.yaml
+
+changing working directory back to "/path/to"