LockFilePatcher should only patch when lock file has holes in it

If not, the export file is stale

TestAssets/LockFileSamples/ExportFileSamples/valid_staleFragment/project.fragment.lock.json

@@ -0,0 +1,35 @@
+{
+  "version": 2,
+  "exports": {
+    "ClassLibrary1/1.0.0": {
+      "type": "project",
+      "framework": ".NETFramework,Version=v4.5.2",
+      "compile": {
+        "bin/Debug/ClassLibrary1.dll": {}
+      },
+      "runtime": {
+        "bin/Debug/ClassLibrary1.dll": {}
+      }
+    },
+    "ClassLibrary2/1.0.0": {
+      "type": "project",
+      "framework": ".NETFramework,Version=v4.6",
+      "compile": {
+        "bin/Debug/ClassLibrary2.dll": {}
+      },
+      "runtime": {
+        "bin/Debug/ClassLibrary2.dll": {}
+      }
+    },
+    "ClassLibrary3/1.0.0": {
+      "type": "project",
+      "framework": ".NETFramework,Version=v4.6",
+      "compile": {
+        "bin/Debug/ClassLibrary3.dll": {}
+      },
+      "runtime": {
+        "bin/Debug/ClassLibrary3.dll": {}
+      }
+    }
+  }
+}

TestAssets/LockFileSamples/ExportFileSamples/valid_staleFragment/project.json

@@ -0,0 +1,10 @@
+{
+    "version": "1.0.0-*",
+    "compilationOptions": {
+        "emitEntryPoint": true
+    },
+
+    "frameworks": {
+      "net46": { }
+    }
+}

TestAssets/LockFileSamples/ExportFileSamples/valid_staleFragment/project.lock.json

@@ -0,0 +1,13 @@
+{
+  "locked": false,
+  "version": 2,
+  "targets": {
+    ".NETFramework,Version=v4.6": { }
+  },
+  "libraries": {
+  },
+  "projectFileDependencyGroups": {
+    "": [],
+    ".NETFramework,Version=v4.6": []
+  }
+}

src/Microsoft.DotNet.ProjectModel/Graph/LockFilePatcher.cs

@@ -26,7 +26,7 @@ namespace Microsoft.DotNet.ProjectModel.Graph
         {
             var exportFilePath = GetExportFilePath(_lockFile.LockFilePath);
 
-            if (File.Exists(exportFilePath))
+            if (File.Exists(exportFilePath) && _msbuildTargetLibraries.Any())
             {
                 var exportFile = LockFileReader.ReadExportFile(exportFilePath);
                 PatchLockWithExport(exportFile);

test/Microsoft.DotNet.ProjectModel.Tests/LockFilePatchingTests.cs

@@ -52,6 +52,21 @@ namespace Microsoft.DotNet.ProjectModel.Tests
             }
         }
 
+        [Fact]
+        public void TestFragmentExistsButNoHolesInLockFile()
+        {
+            var lockFilePath = GetLockFilePath("valid_staleFragment");
+            var lockFile = LockFileReader.Read(lockFilePath);
+
+            var exportFile = lockFile.ExportFile;
+
+            exportFile.Should().BeNull();
+
+            lockFile.Targets.Count.Should().Be(1);
+
+            lockFile.Targets[0].Libraries.Count.Should().Be(0);
+        }
+
         [Fact]
         public void TestMissingExportFileThrows()
         {