Empty "provides" object should not be allowed.

[theory]

Consider this META.json:

{
    "name": "financial",
    "abstract": "Financial aggregate functions",
    "description": "Functions and aggregates for financial calculations",
    "version": "0.0.2",
    "release_status": "testing",

    "maintainer": "Marti Raudsepp <marti@juffo.org>",
    "generated_by": "Marti Raudsepp <marti@juffo.org>",
    "provides": {
    },
    "license": {
        "PostgreSQL": "http://www.postgresql.org/about/licence/"
    },
    "tags": [
        "financial", "finance", "rate of return", "xirr", "aggregate",
        "functions", "excel", "investment"
    ],
    "resources": {
        "bugtracker": {
            "web": "https://bitbucket.org/intgr/pg_financial/issues"
        },
        "repository": {
            "url": "https://bitbucket.org/intgr/pg_financial.git",
            "web": "https://bitbucket.org/intgr/pg_financial",
            "type": "git"
        }
    },

    "meta-spec": {
        "version": "1.0.0",
        "url": "http://pgxn.org/meta/spec.txt"
    }
}

The empty "provides" block results in an empty "Extensions" section in the distribution page. So add code to ensure that the "provides" block is not empty, but includes its required keys.