Maven: skip unresolvable properties

Fixes #14330

Author: yeikel

maven/lib/dependabot/maven/file_parser/property_value_finder.rb

@@ -127,7 +127,7 @@ def resolve_property_placeholder(content, callsite_pom, pom, node, seen_properti
               callsite_pom: callsite_pom,
               seen_properties: seen_properties
             )
-            T.must(resolved)[:value]
+            resolved[:value] if resolved
           end
 
           { file: pom.name, node: node, value: resolved_value }

maven/spec/dependabot/maven/file_parser/property_value_finder_spec.rb

@@ -53,6 +53,13 @@
         its([:value]) { is_expected.to eq("org.reproducer.channels2") }
       end
 
+      context "when the nested property is undefined" do
+        let(:base_pom_fixture_name) { "pom_with_undefined_property.xml" }
+        let(:property_name) { "os.detected.classifier" }
+
+        its([:value]) { is_expected.to eq("") }
+      end
+
       context "when the property name starts with 'project' but not an attribute of the project" do
         let(:base_pom_fixture_name) { "property_name_starts_with_project_pom.xml" }
         let(:property_name) { "project.dependency.spring-boot.version" }

maven/spec/fixtures/poms/pom_with_undefined_property.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>org.reproducer</groupId>
+  <artifactId>test</artifactId>
+  <version>1.0.0-SNAPSHOT</version>
+
+  <properties>
+    <!-- Property intentionally left unresolved in this fixture:
+         ${nisse.os.classifier} is undefined in the test environment.
+         Dependabot should not fail the entire job if this happens
+    -->
+    <os.detected.classifier>${nisse.os.classifier}</os.detected.classifier>
+  </properties>
+
+
+</project>