fix: add error msg to UnmarshalText to make the error clearer

[quagmt]

Description

• Current behavior when unmarshaling empty string into a struct that has a Decimal field is not very clear and can cause confusion. For example:

package main

import (
	"encoding/json"
	"fmt"

	"github.com/quagmt/udecimal"
)

type A struct {
	Amount udecimal.Decimal
}

func main() {
	s := `{"Amount": ""}`
	a := A{}
	err := json.Unmarshal([]byte(s), &a)
	fmt.Println(err)

	// Output:
	// panic: parse empty string
}

• This PR aims to make the error message clearer, instead of just returning parse empty string

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.19%. Comparing base (c313c7c) to head (9a587c9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #36   +/-   ##
=======================================
  Coverage   96.18%   96.19%           
=======================================
  Files           5        5           
  Lines        1889     1892    +3     
=======================================
+ Hits         1817     1820    +3     
  Misses         48       48           
  Partials       24       24           

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