fixed: ProjectProvider.CompilerOptions fails if there are empty references
fixed: ProjectProvider.CompilerOptions fails if there are empty references

isaacabraham/VisualFSharpPowerTools,0x53A/VisualFSharpPowerTools,yever/VisualFSharpPowerTools,dedale/VisualFSharpPowerTools,cloudRoutine/VisualFSharpPowerTools,forki/FSharpVSPowerTools,bentayloruk/FSharpVSPowerTools,enricosada/VisualFSharpPowerTools,fsprojects/VisualFSharpPowerTools,fsprojects/VisualFSharpPowerTools,bleis-tift/FSharpVSPowerTools,cloudRoutine/VisualFSharpPowerTools,smoothdeveloper/VisualFSharpPowerTools,enricosada/VisualFSharpPowerTools,ovatsus/VisualFSharpPowerTools,bleis-tift/FSharpVSPowerTools,dungpa/PowerTools,xperiandri/VisualFSharpPowerTools,vasily-kirichenko/FSharpVSPowerTools,vasily-kirichenko/FSharpVSPowerTools,dkinitiate/VisualFSharpPowerTools