fix(reporter): Prevent null in snippet report template

A `null` value cannot be assigned to a variable in Freemarker. Hence, use
the empty string instead when there is no snippet limit issue.

Signed-off-by: Nicolas Nobelis <[email protected]>

Author: nnobelis

plugins/reporters/fossid/src/main/resources/templates/asciidoc/fossid_snippet.ftl

@@ -33,7 +33,7 @@ List of all the provenances with their files and snippets.
 
 [#assign snippetsLimitIssue = helper.getSnippetsLimitIssue()]
 
-[#if snippetsLimitIssue??]
+[#if snippetsLimitIssue?has_content]
 [WARNING]
 ====
 ${snippetsLimitIssue}

plugins/reporters/freemarker/src/main/kotlin/FreemarkerTemplateProcessor.kt

@@ -279,16 +279,16 @@ class FreemarkerTemplateProcessor(
             input.ortResult.getOpenIssues(minSeverity = threshold).isNotEmpty()
 
         /**
-         * If there are any issue caused by reaching the snippets limit, return the text of the issue. Otherwise reuturn
-         * 'null'.
+         * If there are any issue caused by reaching the snippets limit, return the text of the issue. Otherwise return
+         * the empty string.
          */
         @Suppress("UNUSED") // This function is used in the templates.
         fun getSnippetsLimitIssue() =
             input.ortResult.scanner?.scanResults?.flatMap { result ->
                 result.summary.issues
             }?.firstOrNull {
                 it.message.contains("snippets limit")
-            }?.message
+            }?.message.orEmpty()
 
         /**
          * Return `true` if there are any unresolved and non-excluded [RuleViolation]s whose severity is equal to or