function validatePath {
  Param
  (
      [ValidateScript({
        If ($_ -eq "" -or $_ -eq [String]::Empty) {
            $_ = "C:Install"
            $True
        }
        ElseIf ($_ -match "^([a-z]:\(?:[-\w\.\d])*)") {
            $True
        }
        Else {
            Write-Host "Please enter a valid path,$_ is not a valid path."
            Write-debug $_.Exception
        }
      })]
      [string]$filePath = "C:Install"
  )
  Process
  {
      Write-Host "The path is "$filePath
  }
}

validatePath -filePath $args[0]

Begin{
    If ($filepath -eq "") {
        $filepath = "C:Install"
    }
    ElseIf ($filepath -notmatch "^([a-z]:\(?:[-\w\.\d])*)") {
        Write-Error "Please enter a valid path,$filepath is not a valid path."
    }
}
Process{

function validatePath {
  Param
  (
    [ValidateScript({
      if ($_ -match '^[a-z]:\[-.wd\]*$') { return $True }
      Throw "'$_' is not a valid local path."
    })]
    [string] $filePath = "C:Install"
  )
  Process
  {
    "The path is: $filePath"
  }
}

> validatePath                          # use default value
The path is: C:Install

> validatePath -filePath C:MyInstall   # valid path
The path is: C:MyInstall

> validatePath -filePath NotAFullPath   # invalid path -> error with custom message
validatePath : Cannot validate argument on parameter 'filePath'.
'NotAFullPath' is not a valid local path.
At line:1 char:24
+ validatePath -filePath NotAFullPath   # invalid path
+                        ~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [validatePath], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationError,validatePath

if ([string] $args[0]) { # only true if $args[0] is neither $null nor the empty string
  validatePath -filePath $args[0]
} else {
  validatePath
}

validatePath @args

# Define a hashtable to hold the parameters, if any, to pass through
# to validatePath() via splatting.
$htPassthruParams = @{}

# If the first script argument is neither $null nor the empty string,
# add a hashtable entry for it that will bind to the -filePath parameter.
if ([string] $args[0]) { $htPassthruParams.Add('filePath', $args[0]) }

# Pass the hashtable with `@`, the splatting operator, to validatePath()
validatePath @htPassthruParams

# Define a hashtable to hold the parameters, if any, to pass through
# to validatePath() via splatting.
$htPassthruParams = @{}

# Using a list of parameters, pass their values through only if they are
# *bound*, i.e., only if they received values when the enclosing script/function
# itself was called.
# Assume that the enclosing script declared a -filePath parameter too.
foreach($paramName in , 'filePath') {
  if ($PSBoundParameters.ContainsKey($paramName)) {
    $htPassthruParams.Add($paramName, $PSBoundParameters[$paramName])
  }
}

# Pass the hashtable with `@`, the splatting operator, to validatePath()
validatePath @htPassthruParams