resources: Return nil when not found in resources.Get

Fixes #5999

hugolib/resource_chain_test.go

@@ -356,6 +356,11 @@ CSV2: {{ $csv2 }}
 				`CSV2: [[a b c]]`,
 			)
 		}},
+		{"resources.Get", func() bool { return true }, func(b *sitesBuilder) {
+			b.WithTemplates("home.html", `NOT FOUND: {{ if (resources.Get "this-does-not-exist") }}FAILED{{ else }}OK{{ end }}`)
+		}, func(b *sitesBuilder) {
+			b.AssertFileContent("public/index.html", "NOT FOUND: OK")
+		}},
 
 		{"template", func() bool { return true }, func(b *sitesBuilder) {}, func(b *sitesBuilder) {
 		}},

resources/resource.go

@@ -200,6 +200,9 @@ func (r *Spec) newResource(sourceFs afero.Fs, fd ResourceSourceDescriptor) (reso
 		var err error
 		fi, err = sourceFs.Stat(fd.SourceFilename)
 		if err != nil {
+			if os.IsNotExist(err) {
+				return nil, nil
+			}
 			return nil, err
 		}
 		sourceFilename = fd.SourceFilename