Create diagnostics for ENV and LABEL instructions without a name

[rcjsuen]

If an ENVor LABEL instruction using the name=value syntax doesn't specify a name, an error will be created by the builder. We should create a diagnostic for this same error.

FROM scratch
ENV =y

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : ENV  y
ENV names can not be blank

FROM scratch
LABEL =y

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : LABEL  y
LABEL names can not be blank