Fixed empty version tag for Nuget bug (#156) (#157)

Updated generate nuget tag code to do not provide version tag for empty or null versioning dependency modules/scripts

Author: wooseung-sim

PowerShellGet/PSModule.psm1

@@ -8536,7 +8536,14 @@ function Publish-PSArtifactUtility
             $VersionString = "$($Dependency.MinimumVersion)"
         }
 
-        $dependencies += "<dependency id='$($ModuleName)' version='$($VersionString)' />"
+        if ([System.string]::IsNullOrWhiteSpace($VersionString))
+        {
+            $dependencies += "<dependency id='$($ModuleName)'/>"
+        }
+        else 
+        {
+            $dependencies += "<dependency id='$($ModuleName)' version='$($VersionString)' />"
+        }
     }
 
     # Populate the nuspec elements
@@ -15223,4 +15230,4 @@ Export-ModuleMember -Function Find-Module, `
                     -Alias    fimo, `
                               inmo, `
                               upmo, `
-                              pumo
+                              pumo