Fix inputs with empty string

[MiguelSavignano]

Fix read value from GITHUB_SHA or INPUT_FROM_BRANCH, checking empty string values.

Examples:

name: Merge any release branch to uat
on:
  push:
    branches:
      - 'release/*'
jobs:
  merge-branch:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@master

      - name: Merge staging -> uat
        uses: devmasx/merge-branch@v1.3.0
        with:
          type: now
          target_branch: uat
          github_token: ${{ github.token }}

or

name: Sync multiple branches
on:
  push:
    branches:
      - '*'
jobs:
  sync-branch:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@master

      - name: Merge development -> staging
        uses: devmasx/merge-branch@v1.3.0
        with:
          type: now
          from_branch: development
          target_branch: staging
          github_token: ${{ github.token }}