Don't emit rpm spec fields if empty in dalec spec

This PR prevents the fields `%post`, `%preun`, and `%postun` from being
written to the rpm SPEC unless they are specified in the dalec spec.

This is a short-term solution to the problem specified in Azure#298. Please
see that issue for more details on the long-term solution. A short
summary of the problem follows:

What is happening is that the presence of the `%post`, `%preun`, or
`%postun` causes `/bin/sh` to be baked into the dependencies of the rpm.
This makes sense because a shell is needed to execute the postinstall
scripts, and would be needed to run pre- or post- uninstall scripts.

_without %post_:
```
$ rpm -q --requires /tmp/out/RPMS/x86_64/oras-v1.2.0-1.cm2.x86_64.rpm
rpmlib(CompressedFileNames) <= 3.0.4-1
rpmlib(FileDigests) <= 4.6.0-1
rpmlib(PayloadFilesHavePrefix) <= 4.0-1
rpmlib(PayloadIsZstd) <= 5.4.18-1
```

_with %post_:
```
$ rpm -q --requires /tmp/out/RPMS/x86_64/oras-v1.2.0-1.cm2.x86_64.rpm
/bin/sh
/bin/sh
/bin/sh
rpmlib(CompressedFileNames) <= 3.0.4-1
rpmlib(FileDigests) <= 4.6.0-1
rpmlib(PayloadFilesHavePrefix) <= 4.0-1
rpmlib(PayloadIsZstd) <= 5.4.18-1
```

The `bash` package supplies `/bin/sh`, and all of its dependencies are
installed into the container as well. So the distroless minimal image is
used, but it has a bunch of extra stuff installed.

Signed-off-by: Peter Engelbert <pmengelbert@gmail.com>

frontend/rpm/template.go

@@ -293,45 +293,66 @@ func (w *specWrapper) BuildSteps() fmt.Stringer {
 
 func (w *specWrapper) PreUn() fmt.Stringer {
 	b := &strings.Builder{}
-	b.WriteString("%preun\n")
 
-	if w.Spec.Artifacts.Systemd != nil {
-		keys := dalec.SortMapKeys(w.Spec.Artifacts.Systemd.Units)
-		for _, servicePath := range keys {
-			serviceName := filepath.Base(servicePath)
-			fmt.Fprintf(b, "%%systemd_preun %s\n", serviceName)
-		}
+	if w.Spec.Artifacts.Systemd == nil {
+		return b
+	}
+
+	if len(w.Spec.Artifacts.Systemd.Units) == 0 {
+		return b
+	}
+
+	b.WriteString("%preun\n")
+	keys := dalec.SortMapKeys(w.Spec.Artifacts.Systemd.Units)
+	for _, servicePath := range keys {
+		serviceName := filepath.Base(servicePath)
+		fmt.Fprintf(b, "%%systemd_preun %s\n", serviceName)
 	}
 
 	return b
 }
 
 func (w *specWrapper) Post() fmt.Stringer {
 	b := &strings.Builder{}
+
+	if w.Spec.Artifacts.Systemd == nil {
+		return b
+	}
+
+	if len(w.Spec.Artifacts.Systemd.Units) == 0 {
+		return b
+	}
+
 	b.WriteString("%post\n")
 	// TODO: can inject other post install steps here in the future
 
-	if w.Spec.Artifacts.Systemd != nil {
-		keys := dalec.SortMapKeys(w.Spec.Artifacts.Systemd.Units)
-		for _, servicePath := range keys {
-			unitConf := w.Spec.Artifacts.Systemd.Units[servicePath].Artifact()
-			fmt.Fprintf(b, "%%systemd_post %s\n", unitConf.ResolveName(servicePath))
-		}
+	keys := dalec.SortMapKeys(w.Spec.Artifacts.Systemd.Units)
+	for _, servicePath := range keys {
+		unitConf := w.Spec.Artifacts.Systemd.Units[servicePath].Artifact()
+		fmt.Fprintf(b, "%%systemd_post %s\n", unitConf.ResolveName(servicePath))
 	}
+
 	return b
 }
 
 func (w *specWrapper) PostUn() fmt.Stringer {
 	b := &strings.Builder{}
+
+	if w.Spec.Artifacts.Systemd == nil {
+		return b
+	}
+
+	if len(w.Spec.Artifacts.Systemd.Units) == 0 {
+		return b
+	}
+
 	b.WriteString("%postun\n")
-	if w.Spec.Artifacts.Systemd != nil {
-		keys := dalec.SortMapKeys(w.Spec.Artifacts.Systemd.Units)
-		for _, servicePath := range keys {
-			cfg := w.Spec.Artifacts.Systemd.Units[servicePath]
-			a := cfg.Artifact()
-			serviceName := a.ResolveName(servicePath)
-			fmt.Fprintf(b, "%%systemd_postun %s\n", serviceName)
-		}
+	keys := dalec.SortMapKeys(w.Spec.Artifacts.Systemd.Units)
+	for _, servicePath := range keys {
+		cfg := w.Spec.Artifacts.Systemd.Units[servicePath]
+		a := cfg.Artifact()
+		serviceName := a.ResolveName(servicePath)
+		fmt.Fprintf(b, "%%systemd_postun %s\n", serviceName)
 	}
 
 	return b