Blob upload_from_string AttributeError when using client argument

[jdimatteo]

Environment details

• OS type and version: variant of Debian Linux
• Python version: 3.8.5
• pip version: 20.1.1
• google-cloud-storage version: 1.31.2

Steps to reproduce

1. create Blob object using from_string without a specified client
2. use Blob upload_from_string with client argument

Code example

from google.cloud import storage
from google.cloud.storage.blob import Blob 

def reproduce_error():
    storage_client = storage.Client()
    blob = Blob.from_string("gs://jdimatteo-gunk/example/upload_from_string_error/reproduce_error.txt")
    blob.upload_from_string("foo", client=storage_client)

def workaround_1():
    storage_client = storage.Client()
    blob = Blob.from_string(
            "gs://jdimatteo-gunk/example/upload_from_string_error/workaround_1.txt",
            client=storage_client)
    blob.upload_from_string("workaround_1\n")

def workaround_2():
    storage_client = storage.Client()
    blob = Blob.from_string("gs://jdimatteo-gunk/example/upload_from_string_error/workaround_2.txt")
    blob.bucket._client = storage_client
    blob.upload_from_string("workaround_2\n")

workaround_1()
workaround_2()
reproduce_error()

Stack trace

$ python example.py 
Traceback (most recent call last):
  File "example.py", line 24, in <module>
    reproduce_error()
  File "example.py", line 7, in reproduce_error
    blob.upload_from_string("foo", client=storage_client)
  File "/usr/local/google/home/jdimatteo/gunk/upload_from_string_error_example/venv/lib/python3.8/site-packages/google/cloud/storage/blob.py", line 2440, in upload_from_string
    self.upload_from_file(
  File "/usr/local/google/home/jdimatteo/gunk/upload_from_string_error_example/venv/lib/python3.8/site-packages/google/cloud/storage/blob.py", line 2223, in upload_from_file
    created_json = self._do_upload(
  File "/usr/local/google/home/jdimatteo/gunk/upload_from_string_error_example/venv/lib/python3.8/site-packages/google/cloud/storage/blob.py", line 2055, in _do_upload
    response = self._do_multipart_upload(
  File "/usr/local/google/home/jdimatteo/gunk/upload_from_string_error_example/venv/lib/python3.8/site-packages/google/cloud/storage/blob.py", line 1614, in _do_multipart_upload
    hostname=self.client._connection.API_BASE_URL, bucket_path=self.bucket.path
AttributeError: 'NoneType' object has no attribute '_connection'
$ gsutil cat gs://jdimatteo-gunk/example/upload_from_string_error/workaround_1.txt
workaround_1
$ gsutil cat gs://jdimatteo-gunk/example/upload_from_string_error/workaround_2.txt
workaround_2

[jdimatteo]

Here is an example fix: jdimatteo@df00005

I haven't contributed to this project before and it would take me a little while to get up to speed on the test framework etc.

I'll create a proper pull request if nobody else gets to this before I do.