Add default value for SpaProxyKeepRunning (#45616)

Author: MackinnonBuck

src/Middleware/Spa/SpaProxy/src/build/Microsoft.AspNetCore.SpaProxy.targets

@@ -5,6 +5,7 @@
       <_SpaProxyServerLaunchConfig>$(IntermediateOutputPath)spa.proxy.json</_SpaProxyServerLaunchConfig>
       <_SpaRootFullPath>$([System.IO.Path]::Combine('$(MSBuildProjectDirectory)', '$(SpaRoot)').Replace('\','\\'))</_SpaRootFullPath>
       <SpaProxyTimeoutInSeconds Condition="'$(SpaProxyTimeoutInSeconds)' == ''" >120</SpaProxyTimeoutInSeconds>
+      <SpaProxyKeepRunning Condition="'$(SpaProxyKeepRunning)' == ''" >false</SpaProxyKeepRunning>
     </PropertyGroup>
     <ItemGroup>
       <_SpaProxyServerLaunchConfigLines Include="{" />