Bring back net45 target

src/Markdig.Tests/Markdig.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-  	<TargetFrameworks>netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
+  	<TargetFrameworks>net452;netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
     <OutputType>Library</OutputType>
     <IsPackable>false</IsPackable>
   </PropertyGroup>

src/Markdig/Markdig.targets

@@ -6,7 +6,7 @@
     <NeutralLanguage>en-US</NeutralLanguage>
     <VersionPrefix>0.20.0</VersionPrefix>
     <Authors>Alexandre Mutel</Authors>
-    <TargetFrameworks>netstandard2.0;netstandard2.1;netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>net452;netstandard2.0;netstandard2.1;netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
     <PackageTags>Markdown CommonMark md html md2html</PackageTags>
     <PackageReleaseNotes>https://github.com/lunet-io/markdig/blob/master/changelog.md</PackageReleaseNotes>
     <PackageLicenseExpression>BSD-2-Clause</PackageLicenseExpression>
@@ -20,6 +20,10 @@
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>
   </PropertyGroup>
 
+  <ItemGroup Condition=" '$(TargetFramework)' == 'net452' ">
+    <PackageReference Include="System.Memory" Version="4.5.4" />
+  </ItemGroup>
+
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
     <PackageReference Include="System.Memory" Version="4.5.4" />
   </ItemGroup>

src/Markdig/Syntax/ContainerBlock.cs

@@ -27,7 +27,11 @@ namespace Markdig.Syntax
         /// <param name="parser">The parser used to create this block.</param>
         protected ContainerBlock(BlockParser parser) : base(parser)
         {
+#if NET452
+            children = new Block[0];
+#else
             children = Array.Empty<Block>();
+#endif
         }
 
         /// <summary>

src/Markdig/Syntax/Inlines/ContainerInline.cs

@@ -99,7 +99,11 @@ namespace Markdig.Syntax.Inlines
         {
             if (FirstChild is null)
             {
+#if NET452
+                return new T[0];
+#else
                 return Array.Empty<T>();
+#endif
             }
             else
             {

src/Markdig/Syntax/MarkdownObjectExtensions.cs

@@ -94,7 +94,11 @@ namespace Markdig.Syntax
                 }
             }
 
+#if NET452
+            return new T[0];
+#else
             return Array.Empty<T>();
+#endif
         }
 
         /// <summary>
@@ -124,7 +128,11 @@ namespace Markdig.Syntax
             }
             else
             {
+#if NET452
+                return new T[0];
+#else
                 return Array.Empty<T>();
+#endif
             }
         }