🐛 ENVAR replaced when defined. (#648)

ENVAR replaced (only) when defined. This prevents replacing variables
not associated with ENVAR. For example, in the addon-analyzer, requested
resources are expressed in the metadata as $(var).

For example:
```
  metadata:
    provider:
      address: localhost:$(PORT)
      initConfig:
      - providerSpecificConfig:
          bundles: /jdtls/java-analyzer-bundle/java-analyzer-bundle.core/target/java-analyzer-bundle.core-1.0.0-SNAPSHOT.jar
          depOpenSourceLabelsFile: /usr/local/etc/maven.default.index
          lspServerPath: /jdtls/bin/jdtls
          mavenSettingsFile: $(maven.settings.path)
      name: java
    resources:
    - fields:
      - key: maven.settings.path
        name: settings
        path: /shared/creds/maven/settings.xml
      selector: identity:kind=maven
  selector: tag:Language=Java || !tag:Language
```
An alternative would be a different syntax. Perhaps $((var)). $[var]
$/maven.settings.path/
The `$(maven.settings.path)` should not be replaced with "" since it's
not an ENVAR.

Signed-off-by: Jeff Ortel <jortel@redhat.com>

addon/injector.go

@@ -62,10 +62,15 @@ func (r *EnvInjector) inject(in any) (out any) {
 			if len(match) < 3 {
 				break
 			}
+			k := match[2]
+			v := r.env[k]
+			if v == "" {
+				break
+			}
 			node = strings.Replace(
 				node,
 				match[0],
-				r.env[match[2]],
+				v,
 				-1)
 		}
 		out = node