Add checks for SolutionDir not being empty in remaining uses

Does anyone even use the .props and .props.user stuff or should we delete that?
Fixes #50

Author: drewcassidy

KSPCommon.props

@@ -84,10 +84,10 @@
 
   <!-- Import a props.user file -->
   <!-- serves the same role as the csproj.user file -->
-  <Import Condition=" Exists('$(SolutionDir)$(SolutionName).props.user') " Project="$(SolutionDir)$(SolutionName).props.user"/>
+  <Import Condition=" '$(SolutionDir)' != '' And Exists('$(SolutionDir)$(SolutionName).props.user') " Project="$(SolutionDir)$(SolutionName).props.user"/>
 
   <!-- Import solution-wide props if it exists -->
-  <Import Condition=" Exists('$(SolutionDir)$(SolutionName).props') " Project="$(SolutionDir)$(SolutionName).props"/>
+  <Import Condition=" '$(SolutionDir)' != '' And Exists('$(SolutionDir)$(SolutionName).props') " Project="$(SolutionDir)$(SolutionName).props"/>
 
   <ItemGroup Condition=" exists('$(MSBuildThisFileDirectory)/include') " >
     <Compile

KSPCommon.targets

@@ -15,13 +15,13 @@
     <KSPBT_GameRootSource>environment variable</KSPBT_GameRootSource>
   </PropertyGroup>
   <ItemGroup>
-    <KSPBT_GameRootCandidate Include="$(SolutionDir)KSP" source="solution directory"/>
+    <KSPBT_GameRootCandidate Include="$(SolutionDir)KSP" source="solution directory" Condition=" '$(SolutionDir)' != '' "/>
     <KSPBT_GameRootCandidate Include="$(ReferencePath)" source="reference path"/>
     <KSPBT_GameRootCandidate Include="$(_KSPBT_SteamGameRoot)" source="steam"/>
   </ItemGroup>
 
   <!-- Look for KSP install in Solution dir -->
-  <PropertyGroup Condition=" '$(KSPBT_GameRoot)' == '' And Exists('$(SolutionDir)KSP/$(KSPBT_GameIdentifier)') ">
+  <PropertyGroup Condition=" '$(KSPBT_GameRoot)' == '' And '$(SolutionDir)' != '' And Exists('$(SolutionDir)KSP/$(KSPBT_GameIdentifier)') ">
     <KSPBT_GameRoot>$(SolutionDir)KSP</KSPBT_GameRoot>
     <KSPBT_GameRootSource>solution directory</KSPBT_GameRootSource>
   </PropertyGroup>